6rd
6rd is a mechanism to facilitate rapid deployment of the IPv6 service across IPv4 infrastructures of Internet service providers (ISPs). It uses stateless address mappings between IPv4 and IPv6 addresses, and transmits IPv6 packets across automatic tunnels that follow the same optimized routes between customer nodes as IPv4 packets.
It has been used for the first large deployment of an IPv6 service with native addresses at the end of 2007 (RFC 5569 ). The standard-track specification of the protocol is in RFC 5969.
Read more about this topic: IPv6 Transition Mechanisms