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{{short description|Network that allows computers to share resources and communicate with each other}}
{{citations needed|date=June 2023}}
{{Network science}}

A '''computer network''' is a set of [[Computer|computer]]s sharing resources located on or provided by [[Node (networking)|network nodes]]. Computers use common [[Communication protocol|communication protocol]]s over digital [[Interconnection|interconnection]]s to communicate with each other. These interconnections are made up of telecommunication network technologies based on physically wired, optical, and wireless [[Engineering:Radio frequency|radio-frequency]] methods that may be arranged in a variety of [[Network topology|network topologies]].

The nodes of a computer network can include [[Engineering:Personal computer|personal computer]]s, [[Server (computing)|servers]], [[Networking hardware|networking hardware]], or other specialized or general-purpose [[Host (network)|hosts]]. They are identified by [[Network address|network address]]es and may have [[Hostname|hostname]]s. Hostnames serve as memorable labels for the nodes and are rarely changed after initial assignment. Network addresses serve for locating and identifying the nodes by communication protocols such as the [[Internet Protocol]].

Computer networks may be classified by many criteria, including the [[Physics:Transmission medium|transmission medium]] used to carry signals, [[Bandwidth (computing)|bandwidth]], communications protocols to organize network traffic, the network size, the topology, [[Network traffic control|traffic control]] mechanisms, and organizational intent.{{Citation needed|date=January 2023}}

Computer networks support many applications and services, such as access to the [[World Wide Web]], [[Engineering:Digital video|digital video]] and [[Digital audio|audio]], shared use of [[File server|application and storage servers]], printers and fax machines, and use of email and instant messaging applications.

== History ==
Computer networking may be considered a branch of [[Computer science|computer science]], [[Computer engineering|computer engineering]], and telecommunications, since it relies on the theoretical and practical application of the related disciplines. Computer networking was influenced by a wide array of technological developments and historical milestones.

* In the late 1950s, a network of computers was built for the U.S. military [[Semi-Automatic Ground Environment]] (SAGE) [[Engineering:Radar|radar]] system<ref>{{cite book|url=https://books.google.com/books?id=RBC2nY1rp5MC&dq=semi+automatic+ground+environment+network&pg=PA399|title=Military Communications: From Ancient Times to the 21st Century|year=2008 |editor-first=Christopher H.|editor-last=Sterling|publisher=[[Company:ABC-Clio|ABC-Clio]]|isbn=978-1-85109-737-1|page=399}}</ref><ref>{{cite book|url=https://books.google.com/books?id=KOY9EAAAQBAJ&dq=semi+automatic+ground+environment+network&pg=PA87|title=A New History of Modern Computing|first1=Thomas|last1=Haigh|first2=Paul E.|last2=Ceruzzi|date=14 September 2021 |publisher=MIT Press|isbn=978-0262542906|pages=87–89}}</ref><ref>{{cite book|url=https://books.google.com/books?id=ZITpBQAAQBAJ&q=semi+automatic+ground+environment|title=AN/FSQ-7: the computer that shaped the Cold War|first=Bernd|last=Ulmann|publisher=[[Company:De Gruyter|De Gruyter]]|date=August 19, 2014|isbn=978-3-486-85670-5}}</ref> using the Bell 101 modem. It was the first commercial [[Modem|modem]] for computers, released by AT&T Corporation in 1958. The modem allowed [[Digital data|digital data]] to be transmitted over regular unconditioned telephone lines at a speed of 110 bits per second (bit/s).
* In 1959, [[Biography:Christopher Strachey|Christopher Strachey]] filed a patent application for [[Time-sharing|time-sharing]] and [[Biography:John McCarthy (computer scientist)|John McCarthy]] initiated the first project to implement time-sharing of user programs at MIT.<ref name="ctsspg3">{{cite book|first=F. J.|last=Corbató|display-authors=et al.|url=http://www.bitsavers.org/pdf/mit/ctss/CTSS_ProgrammersGuide.pdf|title=The Compatible Time-Sharing System A Programmer's Guide]|publisher=MIT Press|year=1963|isbn=978-0-262-03008-3|quote=Shortly after the first paper on time-shared computers by C. Strachey at the June 1959 UNESCO Information Processing conference, H. M. Teager and J. McCarthy at MIT delivered an unpublished paper "Time-shared Program Testing" at the August 1959 ACM Meeting.|access-date=2020-05-26|archive-date=2012-05-27|archive-url=https://web.archive.org/web/20120527174321/http://www.bitsavers.org/pdf/mit/ctss/CTSS_ProgrammersGuide.pdf|url-status=live}}</ref><ref>{{Cite web|title=Computer Pioneers - Christopher Strachey|url=https://history.computer.org/pioneers/strachey.html|access-date=2020-01-23|website=history.computer.org|archive-date=2019-05-15|archive-url=https://web.archive.org/web/20190515062531/https://history.computer.org/pioneers/strachey.html|url-status=live}}</ref><ref>{{Cite web|title=Reminiscences on the Theory of Time-Sharing|url=https://jmc.stanford.edu/computing-science/timesharing.html|access-date=2020-01-23|website=jmc.stanford.edu|archive-date=2020-04-28|archive-url=https://web.archive.org/web/20200428094453/http://jmc.stanford.edu/computing-science/timesharing.html|url-status=dead}}</ref><ref>{{Cite web|title=Computer - Time-sharing and minicomputers|url=https://www.britannica.com/technology/computer|access-date=2020-01-23|website=Encyclopedia Britannica|language=en|archive-date=2015-01-02|archive-url=https://web.archive.org/web/20150102192452/http://www.britannica.com/EBchecked/topic/130429/computer/216032/Invention-of-the-modern-computer|url-status=live}}</ref> Strachey passed the concept on to [[Biography:J. C. R. Licklider|J. C. R. Licklider]] at the inaugural [[Biography:International Federation for Information Processing#History|UNESCO Information Processing Conference]] in Paris that year.<ref>{{Cite book|last1=Gillies|first1=James M.|url=https://archive.org/details/howwebwasbornsto00gill|title=How the Web was Born: The Story of the World Wide Web|last2=Gillies|first2=James|last3=Gillies|first3=James and Cailliau Robert|last4=Cailliau|first4=R.|date=2000|publisher=Oxford University Press|isbn=978-0-19-286207-5|pages=[https://archive.org/details/howwebwasbornsto00gill/page/13 13]|language=en|url-access=registration}}</ref> McCarthy was instrumental in the creation of three of the earliest time-sharing systems (the [[Software:Compatible Time-Sharing System|Compatible Time-Sharing System]] in 1961, the [[Software:BBN Time-Sharing System|BBN Time-Sharing System]] in 1962, and the [[Software:Dartmouth Time Sharing System|Dartmouth Time Sharing System]] in 1963).
* In 1959, Anatoly Kitov proposed to the Central Committee of the Communist Party of the Soviet Union a detailed plan for the re-organisation of the control of the Soviet armed forces and of the Soviet economy on the basis of a network of computing centres.<ref>{{Cite web|last=Kitova|first=O|others=Translated by Alexander Nitusov|title=Kitov Anatoliy Ivanovich. Russian Virtual Computer Museum|url=https://www.computer-museum.ru/english/galglory_en/Kitov.htm|access-date=2021-10-11|website=computer-museum.ru|archive-date=2023-02-04|archive-url=https://web.archive.org/web/20230204143509/https://www.computer-museum.ru/english/galglory_en/Kitov.htm|url-status=live}}</ref> Kitov's proposal was rejected, as later was the 1962 [[OGAS]] economy management network project.<ref>{{Cite book |isbn = 978-0262034180|title = How Not to Network a Nation: The Uneasy History of the Soviet Internet|last1 = Peters|first1 = Benjamin|date = 25 March 2016| publisher=MIT Press }}</ref>
* In 1960, the commercial airline reservation system semi-automatic business research environment (SABRE) went online with two connected [[Mainframe computer|mainframes]].
* In 1963, J. C. R. Licklider sent a memorandum to office colleagues discussing the concept of the "[[Intergalactic Computer Network]]", a computer network intended to allow general communications among computer users.
* Throughout the 1960s, Paul Baran and Donald Davies independently developed the concept of [[Packet switching|packet switching]] to transfer information between computers over a network.<ref>{{cite book|last1=Isaacson|first1=Walter|url=https://books.google.com/books?id=4V9koAEACAAJ&pg=PA245|title=The Innovators: How a Group of Hackers, Geniuses, and Geeks Created the Digital Revolution|date=2014|publisher=Simon and Schuster|isbn=9781476708690|pages=237–246|access-date=2021-06-04|archive-date=2023-02-04|archive-url=https://web.archive.org/web/20230204143508/https://books.google.com/books?id=4V9koAEACAAJ&pg=PA245|url-status=live}}</ref><ref>{{cite web|title=NIHF Inductee Paul Baran, Who Invented Packet Switching|url=https://www.invent.org/inductees/paul-baran|access-date=2022-02-12|publisher=National Inventors Hall of Fame|archive-date=2022-02-12|archive-url=https://web.archive.org/web/20220212204116/https://www.invent.org/inductees/paul-baran|url-status=live}}</ref><ref>{{cite web|title=NIHF Inductee Donald Davies, Who Invented Packet Switching|url=https://www.invent.org/inductees/donald-watts-davies|access-date=2022-02-12|publisher=National Inventors Hall of Fame|archive-date=2022-02-12|archive-url=https://web.archive.org/web/20220212204117/https://www.invent.org/inductees/donald-watts-davies|url-status=live}}</ref> Davies pioneered the implementation of the concept. The [[NPL network]], a [[Engineering:Local area network|local area network]] at the [[Organization:National Physical Laboratory (United Kingdom)|National Physical Laboratory (United Kingdom)]] used a line speed of 768 kbit/s and later high-speed [[T-carrier|T1]] links (1.544 Mbit/s line rate).<ref name=":232">{{cite journal|last=Roberts|first=Lawrence G.|date=November 1978|title=The evolution of packet switching|url=https://www.ece.ucf.edu/~yuksem/teaching/nae/reading/1978-roberts.pdf|journal=Proceedings of the IEEE|volume=66|issue=11|pages=1307–13|doi=10.1109/PROC.1978.11141|quote=Both Paul Baran and Donald Davies in their original papers anticipated the use of T1 trunks|s2cid=26876676|access-date=2022-02-12|archive-date=2023-02-04|archive-url=https://web.archive.org/web/20230204143508/https://www.ece.ucf.edu/~yuksem/teaching/nae/reading/1978-roberts.pdf|url-status=live}}</ref><ref name=":32">{{Cite journal|last=Cambell-Kelly|first=Martin|date=1987|title=Data Communications at the National Physical Laboratory (1965-1975)|url=https://archive.org/details/DataCommunicationsAtTheNationalPhysicalLaboratory|journal=Annals of the History of Computing|language=en|volume=9|issue=3/4|pages=221–247|doi=10.1109/MAHC.1987.10023|quote=Transmission of packets of data over the high-speed lines|s2cid=8172150}}</ref><ref>{{Cite news|author=Guardian Staff|date=2013-06-25|title=Internet pioneers airbrushed from history|language=en-GB|work=The Guardian|url=https://www.theguardian.com/technology/2013/jun/25/internet-pioneers-airbrushed-from-history|access-date=2020-07-31|issn=0261-3077|quote=This was the first digital local network in the world to use packet switching and high-speed links.|archive-date=2020-01-01|archive-url=https://web.archive.org/web/20200101062943/https://www.theguardian.com/technology/2013/jun/25/internet-pioneers-airbrushed-from-history|url-status=live}}</ref>
* In 1965, [[Company:Western Electric|Western Electric]] introduced the first widely used telephone switch that implemented computer control in the switching fabric.
* In 1969, the first four nodes of the [[Engineering:ARPANET|ARPANET]] were connected using 50&nbsp;kbit/s circuits between the University of California at Los Angeles, the Stanford Research Institute, the University of California at Santa Barbara, and the University of Utah.<ref>{{cite web|author=Chris Sutton|title=Internet Began 35 Years Ago at UCLA with First Message Ever Sent Between Two Computers|url=https://www.engineer.ucla.edu/stories/2004/Internet35.htm|archive-url=https://web.archive.org/web/20080308120314/https://www.engineer.ucla.edu/stories/2004/Internet35.htm|archive-date=2008-03-08|url-status=dead|publisher=UCLA}}</ref> In the early 1970s, [[Biography:Leonard Kleinrock|Leonard Kleinrock]] carried out mathematical work to model the performance of packet-switched networks, which underpinned the development of the ARPANET.<ref name="Gillies20003">{{cite book|last1=Gillies|first1=James|url=https://archive.org/details/howwebwasbornsto00gill|title=How the Web was Born: The Story of the World Wide Web|last2=Cailliau|first2=Robert|date=2000|publisher=Oxford University Press|isbn=0192862073|page=[https://archive.org/details/howwebwasbornsto00gill/page/25 25]|url-access=registration}}</ref><ref name="Hempstead20053">{{cite book|author1=C. Hempstead|url=https://books.google.com/books?id=2ZCNAgAAQBAJ&pg=PA574|title=Encyclopedia of 20th-Century Technology|author2=W. Worthington|date=2005|publisher=[[Company:Routledge|Routledge]]|isbn=9781135455514|access-date=2017-10-01|archive-date=2023-02-04|archive-url=https://web.archive.org/web/20230204143510/https://books.google.com/books?id=2ZCNAgAAQBAJ&pg=PA574|url-status=live}}</ref> His theoretical work on [[Hierarchical routing|hierarchical routing]] in the late 1970s with student Farouk Kamoun remains critical to the operation of the Internet today.
* In 1972, commercial services were first deployed on public data networks in Europe,<ref>{{cite conference|last=Alarcia|first=G.|author2=Herrera, S.|year=1974|title=C.T.N.E.'s PACKET SWITCHING NETWORK. ITS APPLICATIONS|url=https://rogerdmoore.ca/PS/CTNEA/CTA.html|pages=163–170|book-title=Proceedings of 2nd ICCC 74|archive-url=https://web.archive.org/web/20210414041628/https://rogerdmoore.ca/PS/CTNEA/CTA.html|archive-date=2021-04-14}}</ref><ref>{{cite conference|last=Cuenca|first=L.|year=1980|title=A public packet switching data communications network: eight years of operating experience|url=https://rogerdmoore.ca/PS/CTNEC1.html|publisher=IEEE|pages=39.3.1–39.3.5|book-title=Conference Record of ICC 80|archive-url=https://web.archive.org/web/20210414040737/https://rogerdmoore.ca/PS/CTNEC1.html|archive-date=2021-04-14}}</ref><ref>{{cite conference|last=Lavandera|first=Luis|year=1980|title=ARCHITECTURE, PROTOCOLS AND PERFORMANCE OF RETD|url=https://rogerdmoore.ca/PS/RETDB.html|publisher=IEEE|pages=28.4.1–28.4.5|book-title=Conference Record of ICC 80|archive-url=https://web.archive.org/web/20210414020650/https://rogerdmoore.ca/PS/RETDB.html|archive-date=2021-04-14}}</ref> which began using [[X.25]] in the late 1970s and spread across the globe.<ref name=":232" /> The underlying infrastructure was used for expanding TCP/IP networks in the 1980s.<ref>{{Cite book|last1=Council|first1=National Research|url=https://books.google.com/books?id=Jh1pORpfvrQC&pg=PA148|title=The Unpredictable Certainty: White Papers|last2=Sciences|first2=Division on Engineering and Physical|last3=Board|first3=Computer Science and Telecommunications|last4=Applications|first4=Commission on Physical Sciences, Mathematics, and|last5=Committee|first5=NII 2000 Steering|date=1998-02-05|publisher=National Academies Press|isbn=978-0-309-17414-5|language=en|access-date=2021-03-08|archive-date=2023-02-04|archive-url=https://web.archive.org/web/20230204143512/https://books.google.com/books?id=Jh1pORpfvrQC&pg=PA148|url-status=live}}</ref>
* In 1973, the French [[CYCLADES]] network, directed by Louis Pouzin was the first to make the hosts responsible for the reliable delivery of data, rather than this being a centralized service of the network itself.<ref>{{cite web|last1=Bennett|first1=Richard|title=Designed for Change: End-to-End Arguments, Internet Innovation, and the Net Neutrality Debate|url=https://www.itif.org/files/2009-designed-for-change.pdf|publisher=Information Technology and Innovation Foundation|access-date=2017-09-11|page=11|date=September 2009|archive-date=2019-08-29|archive-url=https://web.archive.org/web/20190829092926/http://www.itif.org/files/2009-designed-for-change.pdf|url-status=dead}}</ref>
* In 1973, Peter Kirstein put [[Internetworking|internetworking]] into practice at [[Organization:University College London|University College London]] (UCL), connecting the ARPANET to British academic networks, the first international heterogeneous computer network.<ref name="Early experiences with the Arpanet">{{cite journal |last1=Kirstein |first1=P.T. |date=1999 |title=Early experiences with the Arpanet and Internet in the United Kingdom |journal=IEEE Annals of the History of Computing |volume=21 |issue=1 |pages=38–44 |doi=10.1109/85.759368 |s2cid=1558618}}</ref><ref>{{Cite journal |last=Kirstein |first=Peter T. |date=2009 |title=The early history of packet switching in the UK |journal=IEEE Communications Magazine |volume=47 |issue=2 |pages=18–26 |doi=10.1109/MCOM.2009.4785372 |s2cid=34735326}}</ref>
* In 1973, [[Biography:Robert Metcalfe|Robert Metcalfe]] wrote a formal memo at Xerox PARC describing [[Ethernet]], a networking system that was based on the [[ALOHAnet|Aloha network]], developed in the 1960s by [[Biography:Norman Abramson|Norman Abramson]] and colleagues at the University of Hawaii. In July 1976, Robert Metcalfe and David Boggs published their paper "Ethernet: Distributed Packet Switching for Local Computer Networks"<ref>{{cite journal |title=Ethernet: Distributed Packet Switching for Local Computer Networks |author1=Robert M. Metcalfe |author2=David R. Boggs |journal=Communications of the ACM |pages=395–404 |volume=19 |issue=5 |date=July 1976|doi=10.1145/360248.360253 |s2cid=429216 |doi-access=free }}</ref> and collaborated on several patents received in 1977 and 1978.
* In 1974, [[Biography:Vint Cerf|Vint Cerf]], Yogen Dalal, and Carl Sunshine published the [[Transmission Control Protocol]] (TCP) specification, {{IETF RFC|675}}, coining the term ''Internet'' as a shorthand for [[Internetworking|internetworking]].<ref>{{cite IETF |title=Specification of Internet Transmission Control Protocol |rfc=675 |last1=Cerf |first1=Vinton|last2=dalal|first2=Yogen|last3=Sunshine|first3=Carl |date=December 1974 |publisher=[[Organization:Internet Engineering Task Force|IETF]]}}</ref>
* In 1976, John Murphy of Datapoint Corporation created [[ARCNET]], a token-passing network first used to share storage devices.
* In 1977, the first long-distance fiber network was deployed by GTE in Long Beach, California.
* In 1977, [[Xerox Network Systems]] (XNS) was developed by Robert Metcalfe and Yogen Dalal at [[Company:Xerox|Xerox]].<ref name="Pelkey-6.9">{{cite book |last1=Pelkey |first1=James L. |title=Entrepreneurial Capitalism and Innovation: A History of Computer Communications, 1968-1988 |date=2007 |chapter=6.9 {{ndash}} Metcalfe Joins the Systems Development Division of Xerox 1975-1978 |chapter-url=https://www.historyofcomputercommunications.info/Book/6/6.9-MetcalfeJoinsSystemsDevelopmentDivisionXerox75-78.html |access-date=2019-09-05 |archive-date=2023-02-04 |archive-url=https://web.archive.org/web/20230204143520/https://historyofcomputercommunications.info/section/8.9/metcalfe-joins-the-systems-development-division-of-xerox-1975-1978/ |url-status=live }}</ref>
* In 1979, Robert Metcalfe pursued making Ethernet an open standard.<ref name="Spurgeon 2000">{{cite book|last=Spurgeon|first=Charles E.|title=Ethernet The Definitive Guide|url=https://archive.org/details/ethernetdefiniti0000spur|url-access=registration|year=2000|publisher=O'Reilly & Associates|isbn=1-56592-660-9}}</ref>
* In 1980, Ethernet was upgraded from the original 2.94&nbsp;Mbit/s protocol to the 10&nbsp;Mbit/s protocol, which was developed by Ron Crane, Bob Garner, Roy Ogus,<ref>{{Cite web|url=https://www.wband.com/2013/05/introduction-to-ethernet-technologies/|title=Introduction to Ethernet Technologies|publisher=WideBand Products|website=www.wband.com|language=en-US|access-date=2018-04-09|archive-date=2018-04-10|archive-url=https://web.archive.org/web/20180410072256/https://www.wband.com/2013/05/introduction-to-ethernet-technologies/|url-status=live}}</ref> and Yogen Dalal.<ref name="Pelkey-Dalal">{{cite book |last1=Pelkey |first1=James L. |title=Entrepreneurial Capitalism and Innovation: A History of Computer Communications, 1968-1988 |date=2007 |chapter=Yogen Dalal |chapter-url=https://historyofcomputercommunications.info/interviews/yogen-dalal/ |access-date=2023-05-07}}</ref>
*In 1995, the transmission speed capacity for Ethernet increased from 10&nbsp;Mbit/s to 100&nbsp;Mbit/s. By 1998, Ethernet supported transmission speeds of 1&nbsp;Gbit/s. Subsequently, higher speeds of up to 400&nbsp;Gbit/s were added ({{as of | 2018 | lc = on}}). The scaling of Ethernet has been a contributing factor to its continued use.<ref name="Spurgeon 2000" />

==Use==
Computer networks extend interpersonal communications by electronic means with various technologies, such as email, instant messaging, online chat, voice and video telephone calls, and video conferencing. A network allows sharing of network and computing resources. Users may access and use resources provided by devices on the network, such as printing a document on a shared network printer or use of a shared storage device. A network allows sharing of files, data, and other types of information giving authorized users the ability to access information stored on other computers on the network. [[Distributed computing]] uses computing resources across a network to accomplish tasks.

==Network packet==
[[File:Network packet.jpg|thumb|Network Packet]]
Most modern computer networks use protocols based on packet-mode transmission. A [[Network packet|network packet]] is a formatted unit of [[Data|data]] carried by a packet-switched network.

Packets consist of two types of data: control information and user data (payload). The control information provides data the network needs to deliver the user data, for example, source and destination [[Network address|network address]]es, error detection codes, and sequencing information. Typically, control information is found in [[Header (computing)|packet headers]] and [[Trailer (computing)|trailers]], with [[Payload (computing)|payload data]] in between.

With packets, the [[Bandwidth (computing)|bandwidth]] of the transmission medium can be better shared among users than if the network were [[Circuit switching|circuit switched]]. When one user is not sending packets, the link can be filled with packets from other users, and so the cost can be shared, with relatively little interference, provided the link is not overused. Often the route a packet needs to take through a network is not immediately available. In that case, the packet is [[Message queue|queued]] and waits until a link is free.

The physical link technologies of packet networks typically limit the size of packets to a certain [[Maximum transmission unit|maximum transmission unit]] (MTU). A longer message may be fragmented before it is transferred and once the packets arrive, they are reassembled to construct the original message.

==Network topology==
{{further|Network topology}}
[[File:NetworkTopologies.svg|thumb|right|Common network topologies]]
The physical or geographic locations of network nodes and links generally have relatively little effect on a network, but the topology of interconnections of a network can significantly affect its throughput and reliability. With many technologies, such as bus or star networks, a single failure can cause the network to fail entirely. In general, the more interconnections there are, the more robust the network is; but the more expensive it is to install. Therefore, most network diagrams are arranged by their [[Network topology|network topology]] which is the map of logical interconnections of network hosts.

Common topologies are:
* [[Bus network]]: all nodes are connected to a common medium along this medium. This was the layout used in the original [[Ethernet]], called [[10BASE5|10BASE5]] and [[10BASE2|10BASE2]]. This is still a common topology on the [[Data link layer|data link layer]], although modern [[Physical layer|physical layer]] variants use [[Engineering:Point-to-point (telecommunications)|point-to-point]] links instead, forming a star or a tree.
* [[Star network]]: all nodes are connected to a special central node. This is the typical layout found in a small switched Ethernet LAN, where each client connects to a central network switch, and logically in a [[Wireless LAN|wireless LAN]], where each wireless client associates with the central [[Wireless access point|wireless access point]].
* [[Ring network]]: each node is connected to its left and right neighbor node, such that all nodes are connected and that each node can reach each other node by traversing nodes left- or rightwards. [[Token ring]] networks, and the [[Fiber Distributed Data Interface]] (FDDI), made use of such a topology.
* Mesh network: each node is connected to an arbitrary number of neighbors in such a way that there is at least one traversal from any node to any other.
* Fully connected network: each node is connected to every other node in the network.
* [[Tree network]]: nodes are arranged hierarchically. This is the natural topology for a larger Ethernet network with multiple switches and without redundant meshing.

The physical layout of the nodes in a network may not necessarily reflect the network topology. As an example, with FDDI, the network topology is a ring, but the physical topology is often a star, because all neighboring connections can be routed via a central physical location. Physical layout is not completely irrelevant, however, as common ducting and equipment locations can represent single points of failure due to issues like fires, power failures and flooding.

===Overlay network===
[[File:Network Overlay merged.svg|thumb|upright=1.5|A sample overlay network]]
An [[Overlay network|overlay network]] is a virtual network that is built on top of another network. Nodes in the overlay network are connected by virtual or logical links.  Each link corresponds to a path, perhaps through many physical links, in the underlying network. The topology of the overlay network may (and often does) differ from that of the underlying one. For example, many [[Peer-to-peer|peer-to-peer]] networks are overlay networks.  They are organized as nodes of a virtual system of links that run on top of the [[Internet]].<ref name=over>{{citation |author1=D. Andersen |author2=H. Balakrishnan |author3=M. Kaashoek |author4-link=Robert Tappan Morris |author4=R. Morris |url=http://nms.lcs.mit.edu/papers/ron-sosp2001.html |title=Resilient Overlay Networks |date=October 2001 |publisher=[[Organization:Association for Computing Machinery|Association for Computing Machinery]] |access-date=2011-11-12 |archive-date=2011-11-24 |archive-url=https://web.archive.org/web/20111124125227/http://nms.lcs.mit.edu/papers/ron-sosp2001.html |url-status=live }}</ref>

Overlay networks have been around since the invention of networking when computer systems were connected over telephone lines using [[Modem|modem]]s before any [[Telecommunications network#Data network|data network]] existed.

The most striking example of an overlay network is the Internet itself. The Internet itself was initially built as an overlay on the [[Engineering:Telephone network|telephone network]].<ref name=over /> Even today, each Internet node can communicate with virtually any other through an underlying mesh of sub-networks of wildly different topologies and technologies. [[Address Resolution Protocol|Address resolution]] and [[Routing|routing]] are the means that allow mapping of a fully connected IP overlay network to its underlying network.

Another example of an overlay network is a [[Distributed hash table|distributed hash table]], which maps keys to nodes in the network. In this case, the underlying network is an IP network, and the overlay network is a table (actually a [[Associative array|map]]) indexed by keys.

Overlay networks have also been proposed as a way to improve Internet routing, such as through [[Quality of service|quality of service]] guarantees achieve higher-quality [[Engineering:Streaming media|streaming media]]. Previous proposals such as IntServ, DiffServ, and [[IP multicast]] have not seen wide acceptance largely because they require modification of all [[Engineering:Router (computing)|routers]] in the network.{{Citation needed|date=August 2010}}  On the other hand, an overlay network can be incrementally deployed on end-hosts running the overlay protocol software, without cooperation from Internet service providers.  The overlay network has no control over how packets are routed in the underlying network between two overlay nodes, but it can control, for example, the sequence of overlay nodes that a message traverses before it reaches its destination{{Citation needed|date=January 2023}}.

For example, [[Company:Akamai Technologies|Akamai Technologies]] manages an overlay network that provides reliable, efficient content delivery (a kind of [[Engineering:Multicast|multicast]]).  Academic research includes end system multicast,<ref>{{cite web |title= End System Multicast |work= project web site |url= https://esm.cs.cmu.edu/ |publisher= Carnegie Mellon University |access-date=2013-05-25 |url-status= dead |archive-url= https://web.archive.org/web/20050221110350/https://esm.cs.cmu.edu/ |archive-date=2005-02-21 }}</ref> resilient routing and quality of service studies, among others.

== Network links ==
{{further|Data transmission}}
[[File:Network links.png|thumb|Network links]]
The transmission media (often referred to in the literature as the ''[[Physics:Transmission medium|physical medium]]'') used to link devices to form a computer network include [[Engineering:Electrical cable|electrical cable]], [[Physics:Fiber-optic communication|optical fiber]], and free space. In the [[OSI model]], the software to handle the media is defined at layers 1 and 2 — the physical layer and the data link layer.

A widely adopted ''family'' that uses copper and fiber media in [[Engineering:Local area network|local area network]] (LAN) technology are collectively known as Ethernet. The media and protocol standards that enable communication between networked devices over Ethernet are defined by [[IEEE 802.3]].  Wireless LAN standards use radio waves, others use infrared signals as a transmission medium. Power line communication uses a building's power cabling to transmit data.

=== Wired ===
[[File:Fibreoptic.jpg|thumb|upright=0.7|alt=Bundle of glass threads with light emitting from the ends|[[Engineering:Fiber-optic cable|Fiber-optic cable]]s are used to transmit light from one computer/network node to another.]]
The following classes of wired technologies are used in computer networking.

*''[[Engineering:Coaxial cable|Coaxial cable]]'' is widely used for cable television systems, office buildings, and other work-sites for local area networks. Transmission speed ranges from 200 million bits per second to more than 500 million bits per second.{{citation needed|reason=Coax is fundamentally capable of more than this. This must refer to a specific [[Ethernet over coax]] standard, but that's covered in the point below.|date=September 2018}}
*ITU-T [[G.hn]] technology uses existing [[Engineering:Home wiring|home wiring]] ([[Ethernet over coax|coaxial cable]], phone lines and power lines) to create a high-speed local area network.
*''[[Twisted pair]]'' cabling is used for wired [[Ethernet]] and other standards. It typically consists of 4 pairs of copper cabling that can be utilized for both voice and data transmission. The use of two wires twisted together helps to reduce crosstalk and [[Physics:Electromagnetic induction|electromagnetic induction]]. The transmission speed ranges from 2&nbsp;Mbit/s to 10&nbsp;Gbit/s. Twisted pair cabling comes in two forms: unshielded twisted pair (UTP) and shielded twisted-pair (STP). Each form comes in several category ratings, designed for use in various scenarios.

[[File:World map of submarine cables.png|thumb|alt=World map with red and blue lines|2007 map showing submarine optical fiber telecommunication cables around the world]]
*An ''[[Engineering:Optical fiber|optical fiber]]'' is a glass fiber. It carries pulses of light that represent data via lasers and [[Physics:Optical amplifier|optical amplifier]]s. Some advantages of optical fibers over metal wires are very low transmission loss and immunity to electrical interference. Using dense [[Wavelength-division multiplexing|wave division multiplexing]], optical fibers can simultaneously carry multiple streams of data on different wavelengths of light, which greatly increases the rate that data can be sent to up to trillions of bits per second. Optic fibers can be used for long runs of cable carrying very high data rates, and are used for undersea communications cables to interconnect continents. There are two basic types of fiber optics, [[Engineering:Single-mode optical fiber|single-mode optical fiber]] (SMF) and [[Engineering:Multi-mode optical fiber|multi-mode optical fiber]] (MMF).  Single-mode fiber has the advantage of being able to sustain a coherent signal for dozens or even a hundred kilometers. Multimode fiber is cheaper to terminate but is limited to a few hundred or even only a few dozens of meters, depending on the data rate and cable grade.<ref name="1961- 2012">{{Cite book |title=CompTIA Network+ exam guide : (Exam N10-005) |last=Meyers |first=Mike |date=2012 |publisher=McGraw-Hill |isbn=9780071789226 |edition=5th |location=New York |oclc=748332969}}</ref>

=== Wireless ===
[[File: Wireless network.jpg|thumb|right|alt=Black laptop with the router in the background |Computers are very often connected to networks using wireless links.]]
{{Main|Engineering:Wireless network}}
Network connections can be established wirelessly using radio or other electromagnetic means of communication.
*'' Terrestrial [[Physics:Microwave|microwave]]''&nbsp;– Terrestrial microwave communication uses Earth-based transmitters and receivers resembling satellite dishes. Terrestrial microwaves are in the low gigahertz range, which limits all communications to line-of-sight. Relay stations are spaced approximately {{convert|40|mi|km}} apart.
*''[[Astronomy:Communications satellite|Communications satellite]]s''&nbsp;– Satellites also communicate via microwave. The satellites are stationed in space, typically in geosynchronous orbit {{convert|35,400|km|mi|abbr=on}} above the equator. These Earth-orbiting systems are capable of receiving and relaying voice, data, and TV signals.
*''[[Engineering:Cellular network|Cellular network]]s'' use several radio communications technologies. The systems divide the region covered into multiple geographic areas. Each area is served by a low-power [[Transceiver|transceiver]].
*''Radio and [[Spread spectrum|spread spectrum]] technologies''&nbsp;– Wireless LANs use a high-frequency radio technology similar to digital cellular. Wireless LANs use spread spectrum technology to enable communication between multiple devices in a limited area. [[IEEE 802.11]] defines a common flavor of open-standards wireless radio-wave technology known as [[Wi-Fi]].
*''[[Engineering:Free-space optical communication|Free-space optical communication]]'' uses visible or invisible light for communications. In most cases, [[Line-of-sight propagation|line-of-sight propagation]] is used, which limits the physical positioning of communicating devices.
* Extending the Internet to interplanetary dimensions via radio waves and optical means, the [[Interplanetary Internet]].<ref>{{citation |url=https://www.ipnsig.org/reports/ISART9-2000.pdf |archive-url=https://web.archive.org/web/20120113053223/https://www.ipnsig.org/reports/ISART9-2000.pdf |archive-date=2012-01-13 |url-status=dead |title=Interplanetary Internet |publisher=Third Annual International Symposium on Advanced Radio Technologies |author=A. Hooke |date=September 2000 |access-date=2011-11-12}}</ref>
* [[IP over Avian Carriers]] was a humorous April fool's Request for Comments, issued as {{IETF RFC|1149}}. It was implemented in real life in 2001.<ref>{{cite web |url=https://www.blug.linux.no/rfc1149 |title=Bergen Linux User Group's CPIP Implementation |publisher=Blug.linux.no |access-date=2014-03-01 |archive-date=2014-02-15 |archive-url=https://web.archive.org/web/20140215072548/http://www.blug.linux.no/rfc1149/ |url-status=dead}}</ref>

The last two cases have a large [[Round-trip delay time|round-trip delay time]], which gives slow [[Social:Two-way communication|two-way communication]] but does not prevent sending large amounts of information (they can have high throughput).

== Network nodes ==
{{Main|Node (networking)}}
Apart from any physical transmission media, networks are built from additional basic system building blocks, such as [[Engineering:Network interface controller|network interface controller]]s, [[Repeater|repeater]]s, [[Ethernet hub|hubs]], [[Network bridge|bridges]], [[Network switch|switches]], [[Engineering:Router (computing)|routers]], modems, and [[Firewall (computing)|firewalls]]. Any particular piece of equipment will frequently contain multiple building blocks and so may perform multiple functions.

=== Network interfaces ===
{{main|Engineering:Network interface controller}}
[[File: ForeRunnerLE 25 ATM Network Interface (1).jpg|thumb|right|alt=A network interface circuit with a port for ATM|An [[Asynchronous Transfer Mode|ATM]] network interface in the form of an accessory card. A lot of network interfaces are built-in.]]

A network interface controller (NIC) is [[Computer hardware|computer hardware]] that connects the computer to the [[Engineering:Network media|network media]] and has the ability to process low-level network information. For example, the NIC may have a connector for plugging in a cable, or an aerial for wireless transmission and reception, and the associated circuitry.

In Ethernet networks, each NIC has a unique [[MAC address|Media Access Control (MAC) address]]—usually stored in the controller's permanent memory. To avoid address conflicts between network devices, the [[Organization:Institute of Electrical and Electronics Engineers|Institute of Electrical and Electronics Engineers]] (IEEE) maintains and administers MAC address uniqueness. The size of an Ethernet MAC address is six [[Octet (computing)|octets]]. The three most significant octets are reserved to identify NIC manufacturers. These manufacturers, using only their assigned prefixes, uniquely assign the three least-significant octets of every Ethernet interface they produce.

=== Repeaters and hubs ===
{{main|Repeater}}

A repeater is an electronic device that receives a network signal, cleans it of unnecessary noise and regenerates it. The signal is [[Retransmission (data networks)|retransmitted]] at a higher power level, or to the other side of obstruction so that the signal can cover longer distances without degradation. In most twisted-pair Ethernet configurations, repeaters are required for cable that runs longer than 100 meters. With fiber optics, repeaters can be tens or even hundreds of kilometers apart.

Repeaters work on the physical layer of the OSI model but still require a small amount of time to regenerate the signal. This can cause a [[Engineering:Propagation delay|propagation delay]] that affects network performance and may affect proper function. As a result, many network architectures limit the number of repeaters used in a network, e.g., the Ethernet [[5-4-3 rule|5-4-3 rule]].

An Ethernet repeater with multiple ports is known as an [[Ethernet hub]]. In addition to reconditioning and distributing network signals, a repeater hub assists with collision detection and fault isolation for the network. Hubs and repeaters in LANs have been largely obsoleted by modern network switches.

=== Bridges and switches===
{{main|Network bridge|Network switch}}

Network bridges and network switches are distinct from a hub in that they only forward frames to the ports involved in the communication whereas a hub forwards to all ports.<ref name=compnetworking>{{cite web |author=Bradley Mitchell |url=https://compnetworking.about.com/cs/internetworking/g/bldef_bridge.htm |title=bridge – network bridges |website=About.com |url-status=dead |archive-url=https://web.archive.org/web/20080328214940/https://compnetworking.about.com/cs/internetworking/g/bldef_bridge.htm |archive-date=2008-03-28}}</ref> Bridges only have two ports but a switch can be thought of as a multi-port bridge. Switches normally have numerous ports, facilitating a star topology for devices, and for cascading additional switches.

Bridges and switches operate at the [[Data link layer|data link layer]] (layer 2) of the OSI model and bridge traffic between two or more [[Network segment|network segment]]s to form a single local network. Both are devices that forward [[Frame (networking)|frames]] of data between [[Engineering:Computer port (hardware)|ports]] based on the destination MAC address in each frame.<ref>{{cite web
|url=https://www.webopedia.com/TERM/s/switch.html
|title=Define switch.
|website=webopedia
|date=September 1996
|access-date=2008-04-08
|archive-date=2008-04-08
|archive-url=https://web.archive.org/web/20080408102559/http://www.webopedia.com/TERM/S/switch.html
|url-status=live
}}</ref>
They learn the association of physical ports to MAC addresses by examining the source addresses of received frames and only forward the frame when necessary. If an unknown destination MAC is targeted, the device broadcasts the request to all ports except the source, and discovers the location from the reply.

Bridges and switches divide the network's collision domain but maintain a single broadcast domain. Network segmentation through bridging and switching helps break down a large, congested network into an aggregation of smaller, more efficient networks.

=== Routers ===
{{main|Engineering:Router (computing)}}

[[File:Adsl connections.jpg|thumb|right|A typical home or small office router showing the [[ADSL|ADSL]] telephone line and [[Ethernet|Ethernet]] network cable connections]]

A router is an internetworking device that forwards packets between networks by processing the addressing or routing information included in the packet.  The routing information is often processed in conjunction with the [[Routing table|routing table]].  A router uses its routing table to determine where to forward packets and does not require broadcasting packets which is inefficient for very big networks.

=== Modems ===
{{main|Modem}}

Modems (modulator-demodulator) are used to connect network nodes via wire not originally designed for digital network traffic, or for wireless. To do this one or more carrier signals are [[Engineering:Modulation|modulated]] by the digital signal to produce an [[Engineering:Analog signal|analog signal]] that can be tailored to give the required properties for transmission. Early modems modulated [[Engineering:Audio signal|audio signal]]s sent over a standard voice telephone line. Modems are still commonly used for telephone lines, using a [[Digital subscriber line|digital subscriber line]] technology and cable television systems using [[DOCSIS]] technology.

=== Firewalls ===
{{main|Firewall (computing)}}
[[File:Firewalls.jpg|thumb|Firewalls]]

A firewall is a network device or software for controlling network security and access rules. Firewalls are inserted in connections between secure internal networks and potentially insecure external networks such as the Internet. Firewalls are typically configured to reject access requests from unrecognized sources while allowing actions from recognized ones. The vital role firewalls play in network security grows in parallel with the constant increase in [[Cyberattack|cyber attack]]s.

== Communication protocols ==
[[File:Internet layering.svg|250px|thumb|alt=Protocols in relation to the Internet layering scheme.|The TCP/IP model and its relation to common protocols used at different layers of the model]]
[[File:Message flows and Routing.svg|250px|thumb|alt=When a router is present, message flows go down through protocol layers, across to the router, up the stack inside the router, and back down again and is sent on to the final destination where it climbs back up the stack|Message flows between two devices (A-B) at the four layers of the TCP/IP model in the presence of a router (R). Red flows are effective communication paths, black paths are across the actual network links.]]

A [[Communication protocol|communication protocol]] is a set of rules for exchanging information over a network. Communication protocols have various characteristics.  They may be [[Connection-oriented communication|connection-oriented]] or [[Connectionless communication|connectionless]], they may use circuit mode or packet switching, and they may use hierarchical addressing or flat addressing.

In a [[Protocol stack|protocol stack]], often constructed per the OSI model, communications functions are divided up into protocol layers, where each layer leverages the services of the layer below it until the lowest layer controls the hardware that sends information across the media. The use of protocol layering is ubiquitous across the field of computer networking. An important example of a protocol stack is [[HTTP]] (the World Wide Web protocol) running over TCP over IP (the Internet protocols) over [[IEEE 802.11]] (the Wi-Fi protocol). This stack is used between the [[Engineering:Wireless router|wireless router]] and the home user's personal computer when the user is surfing the web.

There are many communication protocols, a few of which are described below.
{{Further|Rate Based Satellite Control Protocol}}

===Common protocols===
==== Internet protocol suite ====
The [[Internet protocol suite]], also called TCP/IP, is the foundation of all modern networking. It offers connection-less and connection-oriented services over an inherently unreliable network traversed by datagram transmission using Internet protocol (IP). At its core, the protocol suite defines the addressing, identification, and routing specifications for [[IPv4|Internet Protocol Version 4]] (IPv4) and for [[IPv6]], the next generation of the protocol with a much enlarged addressing capability. The Internet protocol suite is the defining set of protocols for the Internet.<ref name=Tanenbaum>{{cite book|first=Andrew S.|last=Tanenbaum|title=Computer Networks|edition=4th|publisher=Prentice Hall|date=2003}}</ref>

==== IEEE 802 ====
[[IEEE 802]] is a family of IEEE standards dealing with local area networks and metropolitan area networks. The complete IEEE 802 protocol suite provides a diverse set of networking capabilities. The protocols have a flat addressing scheme. They operate mostly at layers 1 and 2 of the OSI model.

For example, [[Bridging (networking)|MAC bridging]] ([[IEEE 802.1D]]) deals with the routing of Ethernet packets using a [[Spanning Tree Protocol]]. [[IEEE 802.1Q]] describes [[VLAN]]s, and [[IEEE 802.1X]] defines a port-based [[Network Access Control]] protocol, which forms the basis for the authentication mechanisms used in VLANs<ref>{{cite journal |url=https://ieeexplore.ieee.org/servlet/opac?punumber=9018452 |date=February 2020 |doi=10.1109/IEEESTD.2020.9018454 |isbn=978-1-5044-6440-6 |at=7.1.3 Connectivity to unauthenticated systems |access-date=2022-05-09 |archive-date=2023-02-04 |archive-url=https://web.archive.org/web/20230204143515/https://ieeexplore.ieee.org/document/9018454 |url-status=live |title=IEEE Standard for Local and Metropolitan Area Networks--Port-Based Network Access Control |journal=IEEE STD 802.1X-2020 (Revision of IEEE STD 802.1X-2010 Incorporating IEEE STD 802.1Xbx-2014 and IEEE STD 802.1Xck-2018) }}</ref> (but it is also found in WLANs<ref>{{cite journal |url=https://ieeexplore.ieee.org/document/9363693 |date=February 2021 |doi=10.1109/IEEESTD.2021.9363693 |isbn=978-1-5044-7283-8 |at=4.2.5 Interaction with other IEEE 802 layers |access-date=2022-05-09 |archive-date=2022-05-17 |archive-url=https://web.archive.org/web/20220517014920/https://ieeexplore.ieee.org/document/9363693 |url-status=live |title=IEEE Standard for Information Technology--Telecommunications and Information Exchange between Systems - Local and Metropolitan Area Networks--Specific Requirements - Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications |journal=IEEE STD 802.11-2020 (Revision of IEEE STD 802.11-2016) }}</ref>)&nbsp;– it is what the home user sees when the user has to enter a "wireless access key".

===== Ethernet =====
Ethernet is a family of technologies used in wired LANs. It is described by a set of standards together called [[IEEE 802.3]] published by the Institute of Electrical and Electronics Engineers.

===== Wireless LAN =====
Wireless LAN based on the [[IEEE 802.11]] standards, also widely known as WLAN or WiFi, is probably the most well-known member of the [[IEEE 802]] protocol family for home users today. IEEE 802.11 shares many properties with wired Ethernet.

==== SONET/SDH ====
[[File:SONET & SDH.jpg|thumb|SONET & SDH]]
[[Synchronous optical networking]] (SONET) and Synchronous Digital Hierarchy (SDH) are standardized [[Engineering:Multiplexing|multiplexing]] protocols that transfer multiple digital bit streams over optical fiber using lasers. They were originally designed to transport circuit mode communications from a variety of different sources, primarily to support circuit-switched [[Engineering:Digital telephony|digital telephony]]. However, due to its protocol neutrality and transport-oriented features, SONET/SDH also was the obvious choice for transporting [[Asynchronous Transfer Mode]] (ATM) frames.

==== Asynchronous Transfer Mode ====
[[File:Asynchronous Transfer Mode.jpg|thumb|Asynchronous Transfer Mode]]
Asynchronous Transfer Mode (ATM) is a switching technique for telecommunication networks.  It uses asynchronous [[Time-division multiplexing|time-division multiplexing]] and encodes data into small, fixed-sized [[Cell relay|cells]]. This differs from other protocols such as the Internet protocol suite or [[Ethernet]] that use variable-sized packets or [[Ethernet frame|frames]]. ATM has similarities with both circuit and packet switched networking.  This makes it a good choice for a network that must handle both traditional high-throughput data traffic, and real-time, low-latency content such as voice and video. ATM uses a connection-oriented model in which a [[Virtual circuit|virtual circuit]] must be established between two endpoints before the actual data exchange begins.

ATM still plays a role in the [[Engineering:Last mile (telecommunications)|last mile]], which is the connection between an Internet service provider and the home user.<ref>{{cite web|last=Martin|first=Thomas|title=Design Principles for DSL-Based Access Solutions|url=https://www.gsi.dit.upm.es/~legf/Varios/XDSL_MARTI.PDF|archive-url=https://web.archive.org/web/20110722194039/https://www.gsi.dit.upm.es/~legf/Varios/XDSL_MARTI.PDF|archive-date=2011-07-22|url-status=dead}}</ref>{{update inline|date=January 2022}}

==== Cellular standards ====
There are a number of different digital cellular standards, including: Global System for Mobile Communications (GSM), [[General Packet Radio Service]] (GPRS), cdmaOne, [[Engineering:CDMA2000|CDMA2000]], [[Engineering:Evolution-Data Optimized|Evolution-Data Optimized]] (EV-DO), Enhanced Data Rates for GSM Evolution (EDGE), Universal Mobile Telecommunications System (UMTS), [[Digital Enhanced Cordless Telecommunications]] (DECT), [[Engineering:Digital AMPS|Digital AMPS]] (IS-136/TDMA), and Integrated Digital Enhanced Network (iDEN).<ref>{{cite book |last=Paetsch |first=Michael |year=1993 |title=The evolution of mobile communications in the US and Europe: Regulation, technology, and markets |location=Boston, London |publisher=Artech House |isbn=978-0-8900-6688-1}}</ref>

=== Routing ===
[[File:XO classroom network.jpg|thumbnail|Routing calculates good paths through a network for information to take. For example, from node 1 to node 6 the best routes are likely to be 1-8-7-6, 1-8-10-6 or 1-9-10-6, as these are the shortest routes.]]

[[Routing]] is the process of selecting network paths to carry network traffic. Routing is performed for many kinds of networks, including circuit switching networks and packet switched networks.

In packet-switched networks, [[Routing protocol|routing protocol]]s direct [[Packet forwarding|packet forwarding]] through intermediate nodes. Intermediate nodes are typically network hardware devices such as routers, bridges, gateways, firewalls, or switches. General-purpose [[Computer|computer]]s can also forward packets and perform routing, though because they lack specialized hardware, may offer limited performance. The routing process directs forwarding on the basis of [[Routing table|routing table]]s, which maintain a record of the routes to various network destinations. Most routing algorithms use only one network path at a time. [[Multipath routing]] techniques enable the use of multiple alternative paths.

Routing can be contrasted with [[Bridging (networking)|bridging]] in its assumption that [[Network address|network address]]es are structured and that similar addresses imply proximity within the network. Structured addresses allow a single routing table entry to represent the route to a group of devices.  In large networks, the structured addressing used by routers outperforms unstructured addressing used by bridging. Structured IP addresses are used on the Internet. Unstructured MAC addresses are used for bridging on Ethernet and similar local area networks.

== Geographic scale ==
{{Area networks}}
Networks may be characterized by many properties or features, such as physical capacity, organizational purpose, user authorization, access rights, and others. Another distinct classification method is that of the physical extent or geographic scale.

===Nanoscale network===
A nanoscale network has key components implemented at the nanoscale, including message carriers, and leverages physical principles that differ from macroscale communication mechanisms. Nanoscale communication extends communication to very small sensors and actuators such as those found in biological systems and also tends to operate in environments that would be too harsh for other communication techniques.<ref>{{cite book |title=Nanoscale Communication Networks |last=Bush |first=S. F. |isbn=978-1-60807-003-9 |publisher=Artech House |year=2010 }}</ref>

===Personal area network===
A [[Personal area network|personal area network]] (PAN) is a computer network used for communication among computers and different information technological devices close to one person. Some examples of devices that are used in a PAN are personal computers, printers, fax machines, telephones, PDAs, scanners, and video game consoles. A PAN may include wired and wireless devices. The reach of a PAN typically extends to 10 meters.<ref>{{cite web |url=https://searchmobilecomputing.techtarget.com/sDefinition/0,,sid40_gci546288,00.html |title=personal area network (PAN) |author=Margaret Rouse |website=TechTarget |access-date=2011-01-29 |archive-date=2023-02-04 |archive-url=https://web.archive.org/web/20230204143532/https://www.techtarget.com/searchmobilecomputing/definition/personal-area-network |url-status=live }}</ref> A wired PAN is usually constructed with [[Engineering:USB|USB]] and FireWire connections while technologies such as [[Bluetooth]] and infrared communication typically form a wireless PAN.

===Local area network===
A [[Engineering:Local area network|local area network]] (LAN) is a network that connects computers and devices in a limited geographical area such as a home, school, office building, or closely positioned group of buildings. Wired LANs are most commonly based on Ethernet technology.  Other networking technologies such as ITU-T [[G.hn]] also provide a way to create a wired LAN using existing wiring, such as coaxial cables, telephone lines, and power lines.<ref>{{cite web |url=https://www.itu.int/ITU-T/newslog/New+Global+Standard+For+Fully+Networked+Home.aspx |title=New global standard for fully networked home |publisher=ITU |website=ITU-T Newslog |access-date=2011-11-12 |date=2008-12-12 |url-status=dead |archive-url=https://web.archive.org/web/20090221090736/https://www.itu.int/ITU-T/newslog/New+Global+Standard+For+Fully+Networked+Home.aspx |archive-date=2009-02-21 }}</ref>

A LAN can be connected to a [[Wide area network|wide area network]] (WAN) using a router. The defining characteristics of a LAN, in contrast to a WAN, include higher data transfer rates, limited geographic range, and lack of reliance on [[Engineering:Leased line|leased line]]s to provide connectivity.{{citation needed|reason=IEEE ref below is narrowly focused on 100G ethernet|date=August 2022}} Current Ethernet or other IEEE 802.3 LAN technologies operate at data transfer rates up to and in excess of [[100 Gigabit Ethernet|100&nbsp;Gbit/s]],<ref>{{cite web |website=IEEE 802.3 ETHERNET WORKING GROUP |url=https://www.ieee802.org/3/ba/ |title=IEEE P802.3ba 40Gb/s and 100Gb/s Ethernet Task Force |access-date=2011-11-12 |archive-date=2011-11-20 |archive-url=https://web.archive.org/web/20111120020900/http://www.ieee802.org/3/ba/ |url-status=live }}</ref> standardized by IEEE in 2010.

===Home area network===
A home area network (HAN) is a residential LAN used for communication between digital devices typically deployed in the home, usually a small number of personal computers and accessories, such as printers and mobile computing devices. An important function is the sharing of Internet access, often a broadband service through a [[Cable Internet access|cable Internet access]] or [[Digital subscriber line|digital subscriber line]] (DSL) provider.

===Storage area network===
A [[Storage area network|storage area network]] (SAN) is a dedicated network that provides access to consolidated, block-level data storage. SANs are primarily used to make storage devices, such as disk arrays, tape libraries, and optical jukeboxes, accessible to servers so that the storage appears as locally attached devices to the operating system. A SAN typically has its own network of storage devices that are generally not accessible through the local area network by other devices. The cost and complexity of SANs dropped in the early 2000s to levels allowing wider adoption across both enterprise and small to medium-sized business environments.{{citation needed|date=September 2022}}

===Campus area network===
A campus area network (CAN) is made up of an interconnection of LANs within a limited geographical area. The networking equipment (switches, routers) and transmission media (optical fiber, Cat5 cabling, etc.) are almost entirely owned by the campus tenant or owner (an enterprise, university, government, etc.).

For example, a university campus network is likely to link a variety of campus buildings to connect academic colleges or departments, the library, and student residence halls.

===Backbone network===
A backbone network is part of a computer network infrastructure that provides a path for the exchange of information between different LANs or subnetworks.  A backbone can tie together diverse networks within the same building, across different buildings, or over a wide area. When designing a network backbone, network performance and [[Network congestion|network congestion]] are critical factors to take into account.  Normally, the backbone network's capacity is greater than that of the individual networks connected to it.

For example, a large company might implement a backbone network to connect departments that are located around the world. The equipment that ties together the departmental networks constitutes the network backbone. Another example of a backbone network is the [[Internet backbone]], which is a massive, global system of fiber-optic cable and optical networking that carry the bulk of data between [[Wide area network|wide area network]]s (WANs), metro, regional, national and transoceanic networks.

===Metropolitan area network===
A metropolitan area network (MAN) is a large computer network that interconnects users with computer resources in a geographic region of the size of a metropolitan area.

===Wide area network===
A [[Wide area network|wide area network]] (WAN) is a computer network that covers a large geographic area such as a city, country, or spans even intercontinental distances.  A WAN uses a communications channel that combines many types of media such as telephone lines, cables, and airwaves. A WAN often makes use of transmission facilities provided by [[Common carrier|common carrier]]s, such as telephone companies. WAN technologies generally function at the lower three layers of the OSI model: the physical layer, the [[Data link layer|data link layer]], and the [[Network layer|network layer]].

===Enterprise private network===
An enterprise private network is a network that a single organization builds to interconnect its office locations (e.g., production sites, head offices, remote offices, shops) so they can share computer resources.

===Virtual private network===
A [[Virtual private network|virtual private network]] (VPN) is an overlay network in which some of the links between nodes are carried by open connections or virtual circuits in some larger network (e.g., the Internet) instead of by physical wires. The data link layer protocols of the virtual network are said to be tunneled through the larger network. One common application is secure communications through the public Internet, but a VPN need not have explicit security features, such as authentication or content encryption. VPNs, for example, can be used to separate the traffic of different user communities over an underlying network with strong security features.

VPN may have best-effort performance or may have a defined service level agreement (SLA) between the VPN customer and the VPN service provider.

===Global area network===
A global area network (GAN) is a network used for supporting mobile users across an arbitrary number of wireless LANs, satellite coverage areas, etc. The key challenge in mobile communications is handing off communications from one local coverage area to the next. In IEEE Project 802, this involves a succession of terrestrial [[Wireless LAN|wireless LAN]]s.<ref>{{cite web |url=https://grouper.ieee.org/groups/802/20/ |website=IEEE 802.20 — Mobile Broadband Wireless Access (MBWA) |title=IEEE 802.20 Mission and Project Scope |access-date=2011-11-12}}</ref>

== Organizational scope ==
Networks are typically managed by the organizations that own them. Private enterprise networks may use a combination of intranets and extranets. They may also provide network access to the Internet, which has no single owner and permits virtually unlimited global connectivity.

=== Intranet ===
An [[Intranet|intranet]] is a set of networks that are under the control of a single administrative entity.  An intranet typically uses the Internet Protocol and IP-based tools such as web browsers and file transfer applications. The administrative entity limits the use of the intranet to its authorized users. Most commonly, an intranet is the internal LAN of an organization. A large intranet typically has at least one web server to provide users with organizational information.

=== Extranet ===
An [[Extranet|extranet]] is a network that is under the administrative control of a single organization but supports a limited connection to a specific external network.  For example, an organization may provide access to some aspects of its intranet to share data with its business partners or customers.  These other entities are not necessarily trusted from a security standpoint.  The network connection to an extranet is often, but not always, implemented via WAN technology.

=== Internet ===
[[File:Internet map 1024.jpg|thumb|right|Partial map of the Internet based on 2005 data.<ref>{{cite web|url=https://www.opte.org/maps/ |title=Maps |website=The Opto Project |archive-url=https://web.archive.org/web/20050115092114/https://www.opte.org/maps/ |archive-date=2005-01-15 |url-status=dead}}</ref> Each line is drawn between two nodes, representing two [[IP address|IP address]]es. The length of the lines indicates the delay between those two nodes.]]

An internetwork is the connection of multiple different types of computer networks to form a single computer network using higher-layer network protocols and connecting them together using routers.

The [[Internet]] is the largest example of internetwork. It is a global system of interconnected governmental, academic, corporate, public, and private computer networks. It is based on the networking technologies of the Internet protocol suite. It is the successor of the [[Engineering:ARPANET|Advanced Research Projects Agency Network]] (ARPANET) developed by DARPA of the United States Department of Defense. The Internet utilizes copper communications and an [[Engineering:Optical networking|optical networking]] backbone to enable the [[World Wide Web]] (WWW), the [[Internet of things]], video transfer, and a broad range of information services.

Participants on the Internet use a diverse array of methods of several hundred documented, and often standardized, protocols compatible with the Internet protocol suite and the IP addressing system administered by the [[Organization:Internet Assigned Numbers Authority|Internet Assigned Numbers Authority]] and [[Organization:Regional Internet registry|address registries]]. Service providers and large enterprises exchange information about the reachability of their address spaces through the [[Border Gateway Protocol]] (BGP), forming a redundant worldwide mesh of transmission paths.

=== Darknet ===
A [[Darknet|darknet]] is an overlay network, typically running on the Internet, that is only accessible through specialized software. It is an anonymizing network where connections are made only between trusted peers&nbsp;— sometimes called ''friends'' ([[Friend-to-friend|F2F]])<ref>{{Cite journal
| last1 = Mansfield-Devine | first1 = Steve
| title = Darknets
| doi = 10.1016/S1361-3723(09)70150-2
| journal = Computer Fraud & Security
| volume = 2009
| issue = 12
| pages = 4–6
|date=December 2009
}}</ref>&nbsp;— using non-standard protocols and [[Port (computer networking)|ports]].

Darknets are distinct from other distributed [[Peer-to-peer|peer-to-peer]] networks as [[Peer-to-peer file sharing|sharing]] is anonymous (that is, IP addresses are not publicly shared), and therefore users can communicate with little fear of governmental or corporate interference.<ref name=Wood>{{cite journal |last=Wood |first=Jessica |title=The Darknet: A Digital Copyright Revolution |journal=Richmond Journal of Law and Technology |year=2010 |volume=16 |issue=4 |url=https://jolt.richmond.edu/v16i4/article14.pdf |access-date=2011-10-25 |archive-date=2012-04-15 |archive-url=https://web.archive.org/web/20120415112930/http://jolt.richmond.edu/v16i4/article14.pdf |url-status=live }}</ref>

== Network service ==
Network services are applications hosted by servers on a computer network, to [[Service (systems architecture)|provide some functionality]] for members or users of the network, or to help the network itself to operate.

The [[World Wide Web]], E-mail,<ref>{{cite ietf|title=Simple Mail Transfer Protocol|rfc=5321|first=J.|last=Klensin|date=October 2008}}</ref> printing and network file sharing are examples of well-known network services. Network services such as [[Domain Name System]] (DNS) give names for [[IP address|IP]] and [[MAC address]]es (people remember names like ''nm.lan'' better than numbers like {{IPaddr|210.121.67.18}}),<ref name="rfc1035">{{cite ietf|rfc=1035|title=Domain names – Implementation and Specification|first=P.|last=Mockapetris|date= November 1987}}</ref> and [[Dynamic Host Configuration Protocol]] (DHCP) to ensure that the equipment on the network has a valid IP address.<ref>{{cite book |last1=Peterson |first1=L.L. |last2=Davie |first2=B.S. |year=2011 |url=https://books.google.com/books?id=BvaFreun1W8C&pg=PA372 |title=Computer Networks: A Systems Approach |page=372 |edition=5th |publisher=Elsevier |isbn=978-0-1238-5060-7}}</ref>

Services are usually based on a service protocol that defines the format and sequencing of messages between clients and servers of that network service.

== Network performance ==
=== Bandwidth ===
[[Bandwidth (computing)|Bandwidth]] in bit/s may refer to consumed bandwidth, corresponding to achieved [[Throughput|throughput]] or [[Goodput|goodput]], i.e., the average rate of successful data transfer through a communication path. The throughput is affected by processes such as bandwidth shaping, [[Bandwidth management|bandwidth management]], [[Bandwidth throttling|bandwidth throttling]], bandwidth cap and [[Physics:Bandwidth allocation|bandwidth allocation]] (using, for example, [[Bandwidth allocation protocol|bandwidth allocation protocol]] and [[Dynamic bandwidth allocation|dynamic bandwidth allocation]]).

===Network delay===
{{main|Network delay}}
''Network delay'' is a design and performance characteristic of a [[Telecommunications network|telecommunications network]]. It specifies the latency for a bit of data to travel across the network from one [[Communication endpoint|communication endpoint]] to another. Delay may differ slightly, depending on the location of the specific pair of communicating endpoints. Engineers usually report both the maximum and average delay, and they divide the delay into several components, the sum of which is the total delay:
* [[Processing delay]]{{snd}} time it takes a router to process the packet header 
* [[Queuing delay]]{{snd}} time the packet spends in routing queues
* [[Transmission delay]]{{snd}} time it takes to push the packet's bits onto the link
* [[Engineering:Propagation delay|Propagation delay]]{{snd}} time for a signal to propagate through the media

A certain minimum level of delay is experienced by signals due to the time it takes to [[Data transmission|transmit]] a packet serially through a [[Data link|link]]. This delay is extended by more variable levels of delay due to [[Network congestion|network congestion]]. IP network delays can range from less than a microsecond to several hundred milliseconds.

=== Performance metrics ===
The parameters that affect performance typically can include [[Throughput|throughput]], [[Jitter|jitter]], [[Bit error rate|bit error rate]] and latency.

In circuit-switched networks, network performance is synonymous with the [[Engineering:Grade of service|grade of service]]. The number of rejected calls is a measure of how well the network is performing under heavy traffic loads.<ref>{{cite book |url=https://www.com.dtu.dk/teletraffic/handbook/telenook.pdf |url-status=dead |archive-url=https://web.archive.org/web/20070111015452/https://oldwww.com.dtu.dk/teletraffic/handbook/telenook.pdf |archive-date=2007-01-11 |author=ITU-D Study Group 2 |date=June 2006 |title=Teletraffic Engineering Handbook}}</ref> Other types of performance measures can include the level of noise and echo.

In an [[Asynchronous Transfer Mode]] (ATM) network, performance can be measured by line rate, quality of service (QoS), data throughput, connect time, stability, technology, modulation technique, and modem enhancements.<ref>{{cite web|url=https://www.telecommagazine.com/|title=Telecommunications Magazine Online|archive-url=https://web.archive.org/web/20110208160737/http://telecommagazine.com/ |archive-date=2011-02-08|date=January 2003}}</ref>{{verification needed |date=August 2018}}{{full citation needed|date=August 2018}}

There are many ways to measure the performance of a network, as each network is different in nature and design. Performance can also be modeled instead of measured. For example, [[State diagram|state transition diagrams]] are often used to model queuing performance in a circuit-switched network. The network planner uses these diagrams to analyze how the network performs in each state, ensuring that the network is optimally designed.<ref>{{cite web |url=https://cne.gmu.edu/modules/os_perf/std.t.html |title=State Transition Diagrams |access-date=2003-07-13 |url-status=dead |archive-url=https://web.archive.org/web/20031015010139/https://cne.gmu.edu/modules/os_perf/std.t.html |archive-date=2003-10-15 }}</ref>

=== Network congestion ===
[[Network congestion]] occurs when a link or node is subjected to a greater data load than it is rated for, resulting in a deterioration of its quality of service. When networks are congested and queues become too full, packets have to be discarded, and participants must rely on [[Retransmission (data networks)|retransmission]] to maintain reliable communications. Typical effects of congestion include queueing delay, [[Packet loss|packet loss]] or the blocking of new connections.  A consequence of these latter two is that incremental increases in [[Offered load|offered load]] lead either to only a small increase in the network [[Throughput|throughput]] or to a potential reduction in network throughput.

Network protocols that use aggressive retransmissions to compensate for packet loss tend to keep systems in a state of network congestion even after the initial load is reduced to a level that would not normally induce network congestion. Thus, networks using these protocols can exhibit two stable states under the same level of load. The stable state with low throughput is known as ''congestive collapse''.

Modern networks use congestion control, congestion avoidance and [[Network traffic control|traffic control]] techniques where endpoints typically slow down or sometimes even stop transmission entirely when the network is congested to try to avoid congestive collapse. Specific techniques include: [[Exponential backoff|exponential backoff]] in protocols such as 802.11's CSMA/CA and the original Ethernet, window reduction in TCP, and fair queueing in devices such as routers. 

Another method to avoid the negative effects of network congestion is implementing [[Quality of service|quality of service]] priority schemes allowing selected traffic to bypass congestion. Priority schemes do not solve network congestion by themselves, but they help to alleviate the effects of congestion for critical services. A third method to avoid network congestion is the explicit allocation of network resources to specific flows. One example of this is the use of Contention-Free Transmission Opportunities (CFTXOPs) in the ITU-T [[G.hn]] home networking standard.

For the Internet, {{IETF RFC|2914}} addresses the subject of congestion control in detail.

=== Network resilience ===
Network resilience is "the ability to provide and maintain an acceptable level of [[Service (systems architecture)|service]] in the face of [[Engineering:Fault (technology)|faults]] and challenges to normal operation."<ref>{{cite web |url=https://resilinets.org/definitions.html#Resilience |publisher=ResiliNets Research Initiative |title=Definitions: Resilience |access-date=2011-11-12 |archive-date=2020-11-06 |archive-url=https://web.archive.org/web/20201106133722/https://resilinets.org/definitions.html#Resilience |url-status=live }}</ref><!--User:Kvng/RTH-->

== Security ==
{{Main|Computer security}}

Computer networks are also used by [[Security hacker|security hacker]]s to deploy [[Computer virus|computer virus]]es or [[Computer worm|computer worm]]s on devices connected to the network, or to prevent these devices from accessing the network via a [[Denial-of-service attack|denial-of-service attack]].

=== Network security ===
Network Security consists of provisions and policies adopted by the [[Network administrator|network administrator]] to prevent and monitor unauthorized access, misuse, modification, or denial of the computer network and its network-accessible resources.<ref>{{cite book | doi = 10.1007/978-3-540-30176-9_41 | last = Simmonds | first = A |author2=Sandilands, P |author3=van Ekert, L| title = Applied Computing | chapter = An Ontology for Network Security Attacks | s2cid = 2204780 | volume = 3285 | pages = 317–323 | year = 2004 | series = Lecture Notes in Computer Science | isbn = 978-3-540-23659-7 }}</ref> Network security is the authorization of access to data in a network, which is controlled by the network administrator. Users are assigned an ID and password that allows them access to information and programs within their authority.  Network security is used on a variety of computer networks, both public and private, to secure daily transactions and communications among businesses, government agencies, and individuals.

=== Network surveillance ===
Network surveillance is the monitoring of data being transferred over computer networks such as the Internet. The monitoring is often done surreptitiously and may be done by or at the behest of governments, by corporations, criminal organizations, or individuals. It may or may not be legal and may or may not require authorization from a court or other independent agency.

Computer and network surveillance programs are widespread today, and almost all Internet traffic is or could potentially be monitored for clues to illegal activity.

Surveillance is very useful to governments and law enforcement to maintain [[Social:Social control|social control]], recognize and monitor threats, and prevent/investigate criminal activity. With the advent of programs such as the Total Information Awareness program, technologies such as high-speed surveillance computers and biometrics software, and laws such as the Communications Assistance For Law Enforcement Act, governments now possess an unprecedented ability to monitor the activities of citizens.<ref name="us-surveillance-soc">{{cite web |url=https://www.aclu.org/other/us-turning-surveillance-society |title=Is the U.S. Turning Into a Surveillance Society? |publisher=American Civil Liberties Union |access-date=2009-03-13 |archive-date=2017-03-14 |archive-url=https://web.archive.org/web/20170314094152/https://www.aclu.org/other/us-turning-surveillance-society |url-status=live }}</ref>

However, many civil rights and [[Privacy|privacy]] groups—such as Reporters Without Borders, the [[Organization:Electronic Frontier Foundation|Electronic Frontier Foundation]], and the American Civil Liberties Union—have expressed concern that increasing surveillance of citizens may lead to a mass surveillance society, with limited political and personal freedoms. Fears such as this have led to numerous lawsuits such as ''Hepting v. AT&T''.<ref name="us-surveillance-soc" /><ref name="bigger-monster">{{cite web |author1=Jay Stanley |author2=Barry Steinhardt |url=https://www.aclu.org/FilesPDFs/aclu_report_bigger_monster_weaker_chains.pdf |title=Bigger Monster, Weaker Chains: The Growth of an American Surveillance Society |date=January 2003 |publisher=American Civil Liberties Union |access-date=2009-03-13 |archive-date=2022-10-09 |archive-url=https://ghostarchive.org/archive/20221009/https://www.aclu.org/FilesPDFs/aclu_report_bigger_monster_weaker_chains.pdf |url-status=live }}</ref> The hacktivist group [[Organization:Anonymous (group)|Anonymous]] has hacked into government websites in protest of what it considers "draconian surveillance".<ref>{{cite web |url=https://www.zdnet.com/blog/security/anonymous-hacks-uk-government-sites-over-draconian-surveillance/11412 |title=Anonymous hacks UK government sites over 'draconian surveillance' |author=Emil Protalinski |website=ZDNet |date=2012-04-07 |access-date=12 March 2013 |archive-date=2013-04-03 |archive-url=https://web.archive.org/web/20130403054106/http://www.zdnet.com/blog/security/anonymous-hacks-uk-government-sites-over-draconian-surveillance/11412 |url-status=live }}</ref><ref>{{cite web |url=https://www.theguardian.com/technology/2012/apr/20/hacktivists-battle-internet |title=Hacktivists in the frontline battle for the internet |author=James Ball |work=The Guardian |date=2012-04-20 |access-date=2012-06-17 |archive-date=2018-03-14 |archive-url=https://web.archive.org/web/20180314103619/https://www.theguardian.com/technology/2012/apr/20/hacktivists-battle-internet |url-status=live }}</ref>

=== End to end encryption ===
[[End-to-end encryption]] (E2EE) is a digital communications paradigm of uninterrupted protection of data traveling between two communicating parties. It involves the originating party [[Encryption|encrypting]] data so only the intended recipient can decrypt it, with no dependency on third parties. End-to-end encryption prevents intermediaries, such as Internet service providers or [[Social:Application service provider|application service provider]]s, from discovering or tampering with communications. End-to-end encryption generally protects both [[Confidentiality|confidentiality]] and [[Data integrity|integrity]].

Examples of end-to-end encryption include [[HTTPS]] for web traffic, [[Software:Pretty Good Privacy|PGP]] for [[Email|email]], [[Software:Off-the-Record Messaging|OTR]] for [[Social:Instant messaging|instant messaging]], [[ZRTP]] for [[Engineering:Telephony|telephony]], and [[Company:Terrestrial Trunked Radio|TETRA]] for radio.

Typical [[Server (computing)|server]]-based communications systems do not include end-to-end encryption. These systems can only guarantee the protection of communications between [[Client (computing)|clients]] and [[Server (computing)|servers]], not between the communicating parties themselves. Examples of non-E2EE systems are [[Software:Google Talk|Google Talk]], Yahoo Messenger, [[Company:Facebook|Facebook]], and [[Software:Dropbox (service)|Dropbox]]. Some such systems, for example, LavaBit and SecretInk, have even described themselves as offering "end-to-end" encryption when they do not. Some systems that normally offer end-to-end encryption have turned out to contain a [[Backdoor (computing)|back door]] that subverts negotiation of the encryption key between the communicating parties, for example [[Software:Skype|Skype]] or [[Software:Hushmail|Hushmail]].

The end-to-end encryption paradigm does not directly address risks at the endpoints of the communication themselves, such as the [[Exploit (computer security)|technical exploitation]] of [[Client (computing)|clients]], poor quality random number generators, or [[Social:Key escrow|key escrow]]. E2EE also does not address [[Traffic analysis|traffic analysis]], which relates to things such as the identities of the endpoints and the times and quantities of messages that are sent.

=== SSL/TLS ===
{{Main|Transport Layer Security}}

The introduction and rapid growth of e-commerce on the World Wide Web in the mid-1990s made it obvious that some form of authentication and encryption was needed. Netscape took the first shot at a new standard. At the time, the dominant web browser was [[Software:Netscape Navigator|Netscape Navigator]]. Netscape created a standard called secure socket layer (SSL). SSL requires a server with a certificate. When a client requests access to an SSL-secured server, the server sends a copy of the certificate to the client. The SSL client checks this certificate (all web browsers come with an exhaustive list of CA root certificates preloaded), and if the certificate checks out, the server is authenticated and the client negotiates a symmetric-key cipher for use in the session. The session is now in a very secure encrypted tunnel between the SSL server and the SSL client.<ref name="1961- 2012"/>

== Views of networks ==
Users and network administrators typically have different views of their networks. Users can share printers and some servers from a [[Workgroup (computer networking)|workgroup]], which usually means they are in the same geographic location and are on the same LAN, whereas a Network Administrator is responsible to keep that network up and running.  A community of interest has less of a connection of being in a local area and should be thought of as a set of arbitrarily located users who share a set of servers, and possibly also communicate via [[Peer-to-peer|peer-to-peer]] technologies.

Network administrators can see networks from both physical and logical perspectives. The physical perspective involves geographic locations, physical cabling, and the network elements (e.g., routers, bridges and [[Application-level gateway|application layer gateways]]) that interconnect via the transmission media. Logical networks, called, in the TCP/IP architecture, [[Subnetwork|subnets]], map onto one or more transmission media. For example, a common practice in a campus of buildings is to make a set of LAN cables in each building appear to be a common subnet, using VLAN technology.

Both users and administrators are aware, to varying extents, of the trust and scope characteristics of a network. Again using TCP/IP architectural terminology, an [[Intranet|intranet]] is a community of interest under private administration usually by an enterprise, and is only accessible by authorized users (e.g. employees).<ref name="RFC2547">{{cite ietf|rfc=2547|title=BGP/MPLS VPNs|first1=E.|last1=Rosen|first2=Y.|last2=Rekhter|date=March 1999}}</ref>  Intranets do not have to be connected to the Internet, but generally have a limited connection.  An [[Extranet|extranet]] is an extension of an intranet that allows secure communications to users outside of the intranet (e.g. business partners, customers).<ref name="RFC2547" />

Unofficially, the Internet is the set of users, enterprises, and content providers that are interconnected by Internet Service Providers (ISP). From an engineering viewpoint, the Internet is the set of subnets, and aggregates of subnets, that share the registered [[IP address]] space and exchange information about the reachability of those IP addresses using the [[Border Gateway Protocol]]. Typically, the human-readable names of servers are translated to IP addresses, transparently to users, via the directory function of the [[Domain Name System]] (DNS).

Over the Internet, there can be [[Business-to-business|business-to-business (B2B)]], business-to-consumer (B2C) and consumer-to-consumer (C2C) communications. When money or sensitive information is exchanged, the communications are apt to be protected by some form of [[Communications security|communications security]] mechanism.  Intranets and extranets can be securely superimposed onto the Internet, without any access by general Internet users and administrators, using secure Virtual Private Network (VPN) technology.

== Journals and newsletters ==
* ''[https://www.degruyter.com/view/j/comp Open Computer Science]'' (open access journal)

== See also ==
{{cmn|
*[[Software:Comparison of network diagram software|Comparison of network diagram software]]
*[[Cyberspace]]
*[[History of the Internet]]
*[[Philosophy:Information Age|Information Age]]
*[[Information revolution]]
*[[Minimum-Pairs Protocol]]
*[[Network simulation]]
*[[Network planning and design]]
*[[Network traffic control]]
*[[Cloud computing|Cloud]]
*[[Engineering:Network on a chip|Network on a chip]]
}}

== References ==
{{Reflist|30em}}

== Further reading ==
* Shelly, Gary, et al. "Discovering Computers" 2003 Edition.
* Wendell Odom, Rus Healy, Denise Donohue. (2010) CCIE Routing and Switching. Indianapolis, IN: Cisco Press
* Kurose James F and Keith W. Ross: Computer Networking: A Top-Down Approach Featuring the Internet, Pearson Education 2005.
* [[Biography:William Stallings|William Stallings]], ''Computer Networking with Internet Protocols and Technology'', Pearson Education 2004.
* [[List of important publications in computer science#Networking|Important publications in computer networks]]
* Network Communication Architecture and Protocols: OSI Network Architecture 7 Layers Model
* [[Biography:Dimitri Bertsekas|Dimitri Bertsekas]], and Robert Gallager, "Data Networks," Prentice Hall, 1992.

== External links ==
* {{curlie|Computers/Software/Networking/|Networking}}
* [http://standards-oui.ieee.org/oui/oui.txt IEEE Ethernet manufacturer information]
* [https://www.ciena.com/insights/acronym-guide/ A computer networking acronym guide]

{{Operating System}}
{{Computer science}}

[[Category:Computer networks| ]]

{{Sourceattribution|Computer network}}