6to4

6to4 is an IPv6 transition mechanism that allows IPv6 packets to be transmitted over an IPv4 network without requiring explicit tunnels. It was defined in RFC 3056 and became a popular method in the early days of IPv6 deployment, providing connectivity for networks that had only IPv4 addresses.

How it works: A device with a globally routable IPv4 address automatically configures a 6to4 IPv6 address using the 2002::/16 prefix derived from that IPv4 address. For example, an IPv4 address is embedded into the IPv6 prefix to form a 6to4 address such as 2002:XXXX:XXXX::/48. When communicating with a remote IPv6 destination, the IPv6 packet is encapsulated in an IPv4 header (protocol 41) and sent toward a 6to4 relay router. The relay decapsulates the packet and forwards it toward the destination. Routing relies on the embedded IPv4-derived prefix, and traffic typically traverses the IPv4 Internet via 6to4 relay routers.

Addressing and relays: 6to4 addresses begin with 2002:. The relay infrastructure uses 6to4 Relay Routers, often reached via an anycast address such as 192.88.99.1 in practice. If a suitable relay is available, traffic is forwarded to the corresponding IPv6 destination; otherwise connectivity can fail.

Limitations and status: 6to4 depends on functional relay routers and compatible IPv4 paths. NATs, firewalls, and asymmetric routing can block or degrade traffic, and some networks block protocol 41 traffic. Because of reliability and scalability concerns, 6to4 has fallen out of favor for new deployments and is frequently disabled on modern networks. Alternatives for IPv6 transition include native IPv6, dual-stack configurations, Teredo, and 6rd.