Native IPv6 Behind NAT44 CPEs (6a44)
In customer sites having IPv4-only CPEs, Teredo provides a last resort IPv6 connectivity [RFC4380] [RFC5991] [RFC6081]. However, because it is designed to work without involvement of Internet service providers, it has significant limitations (connectivity between IPv6 native addresses and Teredo addresses is uncertain; connectivity between Teredo addresses fails for some combinations of NAT types). 6a44 is a complementary solution that, being base on ISP cooperation, avoids these limitations. At the beginning of IPv6 addresses, it replaces the Teredo well-known prefix by network specific /48 prefixes assigned by local ISP's (an evolution similar to that from 6to4 to 6rd). The specification is complete enough for actual deployment, including with independently written codes.