4to4

4to4 is an IPv6 transition mechanism designed to allow IPv6 traffic to be carried over IPv4 networks without requiring native IPv6 connectivity at every hop. It achieves this by using a dedicated addressing scheme that encodes enough information to locate the remote endpoint and by tunneling IPv6 packets through the IPv4 Internet to a 4to4 relay.

In operation, a 4to4-enabled host encapsulates an IPv6 packet inside an IPv4 packet. The IPv6 destination address is constructed so that it identifies the remote 4to4 endpoint, whose IPv4 address is embedded in the IPv6 address itself. The IPv4 packet is then routed through the IPv4 network to a 4to4 relay or directly to the remote endpoint, which decapsulates the IPv4 envelope and delivers the original IPv6 packet to its destination.

This approach relies on 4to4 relay routers located at strategic points on the IPv4 Internet to forward traffic between 4to4-enabled sites. Each end of a 4to4 tunnel uses the other end’s embedded IPv4 address to determine how to reach the peer, enabling IPv6 communication over an IPv4-only path where native IPv6 is not available.

Adoption of 4to4 peaked in the mid-2000s but has since declined as native IPv6 deployment progressed and alternative transition methods matured. It remains largely obsolete for new deployments due to reliability concerns, dependence on relay availability, and the emergence of more robust techniques such as native dual-stack operation and NAT64/translation approaches.

See also: IPv6 transition mechanisms, 6to4, NAT64.