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{{Short description|IPv6 transition mechanism}}
{{IPv6 transition mechanisms}}
'''6over4''' is an [[IPv6]] [[IPv6#Transition mechanisms|transition mechanism]] meant to transmit IPv6 packets between dual-stack nodes on top of a [[Engineering:Multicast|multicast]]-enabled [[IPv4]] network. IPv4 is used as a virtual [[Data link layer|data link layer]] (''virtual [[Ethernet]]'') on which  IPv6 can be run.

==How 6over4 works==
6over4 defines a trivial method for generating a link-local IPv6 address from an IPv4 address, and a mechanism to perform [[Neighbor Discovery Protocol|Neighbor Discovery]] on top of IPv4.

===Link-local address generation===
Any host wishing to participate in 6over4 over a given IPv4 network can set up a virtual IPv6 network interface. The link-local address is determined as follows :
* it starts with <code>fe80:0000:0000:0000:0000:0000</code>, or <code>fe80::</code> for short,
* the lower-order 32 bits to the binary value must be that of the IPv4 address of the host.

For example, host <code>192.0.2.142</code> would use <code>fe80:0000:0000:0000:0000:0000:c000:028e</code> as its link-local IPv6 address (<code>192.0.2.142</code> is <code>c000028e</code> in [[Hexadecimal|hexadecimal]] notation). A shortened notation would be <code>fe80::c000:028e</code>.

===Multicast Address Mapping===
To perform [[ICMPv6]] Neighbor Discovery, multicast must be used. Any IPv6 multicast packet gets encapsulated in an IPv4 multicast packet with destination <code>239.192.x.y</code>, where <code>x</code> and <code>y</code> are the penultimate and last bytes of the IPv6 multicast address respectively.
====Examples====


All-Nodes Multicast (ff02::1) - <code>239.192.0.1</code>

All-Routers Multicast (ff02::2) - <code>239.192.0.2</code>

Solicited Node Multicast for fe80::c000:028e (the link-local address of 192.0.2.142) - <code>239.192.2.142</code>

===Neighbor Discovery===
Given a link-local address and a multicast addresses mapping, a host can use ICMPv6 to discover  its on-link neighbors and routers, and usually perform stateless autoconfiguration, as it would do on top of, e.g. [[Ethernet]].

==Limit of 6over4==
6over4 relies on IPv4 multicast availability which is not very widely supported by IPv4 networking infrastructure. 6over4 is of limited practical use, and is not supported by the most common [[Operating system|operating system]]s.
To connect IPv6 hosts on different physical links, IPv4 multicast routing must be enabled on the routers connecting the links.It will also work on windows,iOS, Android,etc.

[[ISATAP]] is a more complex alternative to 6over4 which does not rely on IPv4 multicast.

==References==
* B. Carpenter & C. Jung ''Transmission of IPv6 over IPv4 Domains without Explicit Tunnels'' {{IETF RFC|2529}}, March 1999.

== See also ==
* [[4over6|4over6]]

[[Category:IPv6 transition technologies]]

{{Sourceattribution|6over4}}