6in4

6in4, sometimes referred to as SIT,^[a] is an IPv6 transition mechanism for migrating from Internet Protocol version 4 (IPv4) to IPv6. It is a tunneling protocol that encapsulates IPv6 packets on specially configured IPv4 links according to the specifications of RFC 4213. The IP protocol number for 6in4 is 41, per IANA reservation.^[1]

The 6in4 packet format consists of the IPv6 packet preceded by an IPv4 packet header. Thus, the encapsulation overhead is the size of the IPv4 header of 20 bytes. On Ethernet with a maximum transmission unit (MTU) of 1500 bytes, IPv6 packets of 1480 bytes may therefore be transmitted without fragmentation.

6in4 tunneling is also referred to as proto-41 static because the endpoints are configured statically. Although 6in4 tunnels are generally manually configured, the utility AICCU can configure tunnel parameters automatically after retrieving information from a Tunnel Information and Control Protocol (TIC) server.

The similarly named methods 6to4 or 6over4 describe a different mechanism. The 6to4 method also makes use of proto-41, but the endpoint IPv4 address information is derived from the IPv6 addresses within the IPv6 packet header, instead of from static configuration of the endpoints.

Cite error: There are <ref group=lower-alpha> tags or {{efn}} templates on this page, but the references will not show without a {{reflist|group=lower-alpha}} template or {{notelist}} template (see the help page).