6in4 is an Internet transition mechanism for migrating from Internet Protocol version 4 (IPv4) to IPv6. 6in4 uses tunneling to encapsulate IPv6 traffic over explicitly-configured IPv4 links as defined in RFC 4213 (obsoletes RFC 2893 and RFC 1933). The 6in4 traffic is sent over the IPv4 Internet inside IPv4 packets whose IP headers have the IP protocol numberset to 41. This protocol number is specifically designated for IPv6 encapsulation. In 6in4, the IPv4 packet header is immediately followed by the IPv6 packet being carried. This means that the encapsulation overhead is simply the size of the IPv4 header of 20 bytes. With an Ethernet Maximum Transmission Unit (MTU) of 1500 bytes, one can thus send IPv6 packets of 1480 bytes 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, for example the utility AICCU can configure tunnel parameters automatically after retrieving information from a Tunnel Information and Control Protocol (TIC) server.
There are similarly named methods, namely 6to4 or 6over4, which describe a different mechanism. The 6to4 method makes use of proto-41 too, but instead of static configuration of the endpoints, the endpoint IPv4 address information is derived from the IPv6 addresses within the IPv6 packet header.
Generic Routing Encapsulation (GRE) is a tunneling protocol developed by Cisco Systems that can encapsulate a wide variety of network layer protocols inside virtual point-to-point links or point-to-multipoint links over an Internet Protocol network.