6over4 is an IPv6 transition mechanism meant to transmit IPv6 packets between

dual-stack Internet Protocol version 6 (IPv6) is the most recent version of the Internet Protocol (IP), the communications protocol that provides an identification and location system for computers on networks and routes traffic across the Internet. IPv ...

nodes on top of a multicast-enabled

IPv4 Internet Protocol version 4 (IPv4) is the fourth version of the Internet Protocol (IP). It is one of the core protocols of standards-based internetworking methods in the Internet and other packet-switched networks. IPv4 was the first version de ...

network. IPv4 is used as a virtual 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 The Neighbor Discovery Protocol (NDP), or simply Neighbor Discovery (ND), is a protocol of the Internet protocol suite used with Internet Protocol Version 6 (IPv6). It operates at the link layer of the Internet model, and is responsible for ga ...

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 fe80:0000:0000:0000:0000:0000, or fe80:: for short, * the lower-order 32 bits to the binary value must be that of the IPv4 address of the host. For example, host 192.0.2.142 would use fe80:0000:0000:0000:0000:0000:c000:028e as its link-local IPv6 address (192.0.2.142 is c000028e in

hexadecimal In mathematics and computing, the hexadecimal (also base-16 or simply hex) numeral system is a positional numeral system that represents numbers using a radix (base) of 16. Unlike the decimal system representing numbers using 10 symbols, hexa ...

notation). A shortened notation would be fe80::c000:028e.

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 239.192.x.y, where x and y are the penultimate and last bytes of the IPv6 multicast address respectively.

Examples

All-Nodes Multicast (ff02::1) - 239.192.0.1 All-Routers Multicast (ff02::2) - 239.192.0.2 Solicited Node Multicast for fe80::c000:028e (the link-local address of 192.0.2.142) - 239.192.2.142

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 systems. To connect IPv6 hosts on different physical links, IPv4 multicast routing must be enabled on the routers connecting the links. 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.