6to4

6to4 is an automatic tunnelling method for carrying IPv6 packets across IPv4 networks. It is defined in RFC 3056. An IPv6 prefix within 2002::/16 is derived from a public IPv4 address, so each site automatically receives its own /48 prefix. A 6to4 router encapsulates IPv6 packets in IPv4 packets using protocol number 41, and the remote endpoint removes the IPv4 header. Connections to native IPv6 networks require 6to4 relay routers.

6to4 does not require manually configured tunnels. In practice, however, it has limitations related to NAT, asymmetric routing and the reliability of public relays. RFC 7526 deprecated the anycast relay mechanism. Enterprise networks therefore typically rely on dual stack, native IPv6 connectivity or other transition mechanisms today.