6over4 is an IPv6

IPv6

Internet Protocol version 6 is a version of the Internet Protocol . It is designed to succeed the Internet Protocol version 4...

 transition mechanism meant to transmit IPv6 packets between dual-stack nodes on top of a multicast

Multicast

In computer networking, multicast is the delivery of a message or information to a group of destination computers simultaneously in a single transmission from the source creating copies automatically in other network elements, such as routers, only when the topology of the network requires...

-enabled IPv4

IPv4

Internet Protocol version 4 is the fourth revision in the development of the Internet Protocol and the first version of the protocol to be widely deployed. Together with IPv6, it is at the core of standards-based internetworking methods of the Internet...

 network. IPv4 is used as a virtual data link layer

Data link layer

The data link layer is layer 2 of the seven-layer OSI model of computer networking. It corresponds to, or is part of the link layer of the TCP/IP reference model....

 (virtual Ethernet

Ethernet

Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....

) 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

Neighbor Discovery Protocol

The Neighbor Discovery Protocol is a protocol in the Internet Protocol Suite used with Internet Protocol Version 6 . It operates in the Link Layer of the Internet model and is responsible for address autoconfiguration of nodes, discovery of other nodes on the link, determining the Link Layer...

 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

Hexadecimal

In mathematics and computer science, hexadecimal is a positional numeral system with a radix, or base, of 16. It uses sixteen distinct symbols, most often the symbols 0–9 to represent values zero to nine, and A, B, C, D, E, F to represent values ten to fifteen...

 notation). A shortened notation would be fe80::c000:28e.

Multicast Address Mapping

To perform ICMPv6

ICMPv6

Internet Control Message Protocol version 6 is the implementation of the Internet Control Message Protocol for Internet Protocol version 6 defined in RFC 4443...

 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::c000:28e (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

Ethernet

Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....

.

Limit of 6over4

6over4 relies on IPv4 multicast availability, which is not very widely supported by IPv4 networking infrastructure (multicast is almost as recent as IPv6), 6over4 is of limited practical use, and is not supported by the most common operating system

Operating system

An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...

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

ISATAP

ISATAP

ISATAP is an IPv6 transition mechanism meant to transmit IPv6 packets between dual-stack nodes on top of an IPv4 network....

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