The transition mechanism known as 6to4 [RFC3056] is a form of automatic router-to-router tunnelling that uses the IANA-assigned IPv6 prefix 2002::/16 to designate a site that participates in 6to4. It allows isolated IPv6 domains to communicate with other IPv6 domains with minimal configuration. An isolated IPv6 site will assign itself a prefix of 2002:V4ADDR::/48, where V4ADDR is the globally unique IPv4 address configured on the appropriate interface of the domain’s egress router (see Figure5-2). This prefix has exactly the same format as normal /48 prefixes and thus allows an IPv6 domain to use it like any other valid /48 prefix. In the scenario where 6to4 domains wish to communicate with other 6to4 domains, no tunnel configuration is needed. Tunnel endpoints are determined by the value of the global routing prefix of the IPv6 destination address contained in the IPv6 packet being transmitted, which includes the IPv4 address. In this scenario, an arbitrary number of 6to4 domains may communicate without the need for any tunnel configuration. Furthermore, the 6to4 routers do not need to run any exterior IPv6 routing protocol as IPv4 exterior routing performs the task instead.

in-IPv4 encapsulation without explicit tunnels. It uses IPv4 addresses as interface identifiers and creates a virtual link using an IPv4 multicast group with organization-local scope. IPv6 multicast addresses are mapped to IPv4 multicast addresses to allow neighbour discovery. The 6over4 method has fallen out of favour due to a number of reasons, including the general lack of IPv4 multicast support in site/ISP networks.

There have been a small number of implementations, including those by 3Com and Cisco, but practically no adoption. We thus do not consider 6over in any detail, as the method seems (in effect) deprecated. We would thus not recommend its use.