6in4

6in4 is an IPv6 transition mechanism that carries IPv6 packets over an IPv4 network by encapsulating them in IPv4 packets. Defined in RFC 2529, it enables IPv6 connectivity without requiring native IPv6 infrastructure on the underlying network. The IPv4 header carries the IPv6 payload, using IPv4 protocol number 41, and the tunnel endpoints are identified by their public IPv4 addresses.

In operation, each end of a 6in4 tunnel has a configured IPv4 address. The IPv6 address assigned to the tunnel interface is derived from the remote endpoint’s IPv4 address by encoding that address in hexadecimal after the 2002: prefix. A common representation is 2002:<IPv4-address-in-hex>::/64, forming the /64 IPv6 network used for the tunnel. The tunnel transmits IPv6 packets from one site to the other, where they are decapsulated and forwarded onto the IPv6 network at the far end.

Configuration and deployment typically involve either manual setup on routers or the use of a tunnel broker service that supplies the IPv4 endpoints and the corresponding IPv6 prefix. On each side, you configure the tunnel source and destination (the two IPv4 addresses), enable protocol 41 encapsulation, and assign the /64 IPv6 prefix derived from the remote IPv4 address to the tunnel interface. Once established, IPv6 routers behind the tunnel can route within the assigned prefix as if directly connected.

Limitations include reliance on publicly reachable IPv4 addresses, susceptibility to NAT or firewall rules that block protocol 41, and the absence of inherent encryption or authentication. MTU considerations and potential path MTU discovery issues can also affect performance. Security can be enhanced by combining 6in4 with IPsec, but 6in4 itself provides no built-in encryption.