6to4

6to4 is an Internet transition mechanism for migrating from Internet Protocol version 4 (IPv4) to version 6 (IPv6) and a system that allows IPv6 packets to be transmitted over an IPv4 network (generally the IPv4 Internet) without the need to configure explicit tunnels. Special relay servers are also in place that allow 6to4 networks to communicate with native IPv6 networks.^[1]

6to4 is especially relevant during the initial phases of deployment to full, native IPv6 connectivity, since IPv6 is not required on nodes between the host and the destination. However, it is intended only as a transition mechanism and is not meant to be used permanently.

6to4 may be used by an individual host, or by a local IPv6 network. When used by a host, it must have a global IPv4 address connected, and the host is responsible for encapsulation of outgoing IPv6 packets and decapsulation of incoming 6to4 packets. If the host is configured to forward packets for other clients, often a local network, it is then a router.

Most IPv6 networks use autoconfiguration, which requires the last 64 bits of the address for the host. The first 64 bits are the IPv6 prefix. The first 16 bits of the prefix are always 2002:, the next 32 bits are the IPv4 address, and the last 16 bits of the prefix are available for addressing multiple IPv6 subnets behind the same 6to4 router. Since the IPv6 hosts using autoconfiguration already have determined the unique 64 bit host portion of their address, they must simply wait for a Router Advertisement indicating the first 64 bits of prefix to have a complete IPv6 address. A 6to4 router will know to send an encapsulated packet directly over IPv4 if the first 16 bits are 2002, using the next 32 as the destination, or otherwise send the packet to a well-known relay server, which has access to native IPv6.

6to4 does not facilitate interoperation between IPv4-only hosts and IPv6-only hosts (for that, see NAT64). 6to4 is simply a transparent mechanism used as a transport layer between IPv6 nodes.

Due to the high levels of misconfigured hosts and poor performance observed, an advisory about how 6to4 should be deployed was published in August 2011.^[2] Due to unsolvable operational problems using the 6to4 anycast prefix, that part of the standard was deprecated in 2015.^[3]