4in6 refers to tunneling of IPv4

IPv4

Internet Protocol version 4 is the fourth revision in the development of the Internet Protocol and the first version of the protocol to be widely deployed. Together with IPv6, it is at the core of standards-based internetworking methods of the Internet...

 in IPv6

IPv6

Internet Protocol version 6 is a version of the Internet Protocol . It is designed to succeed the Internet Protocol version 4...

. It is an Internet interoperation mechanism allowing Internet Protocol version 4 (IPv4) to be used in an IPv6 only network. 4in6 uses tunneling to encapsulate IPv4 traffic over configured IPv6 tunnels as defined in RFC 2473. 4in6 tunnels are usually manually configured but they can be automated using protocols such as TSP

Tunnel Setup Protocol

In computer networking, the Tunnel Setup Protocol is a networking control protocol used to negotiate IP tunnel setup parameters between a tunnel client host and a tunnel broker server, the tunnel end-points. The protocol is defined in RFC 5572...

 to allow easy connection to a tunnel broker

Tunnel broker

In the context of computer networking, a tunnel broker is a service which provides a network tunnel. These tunnels can provide encapsulated connectivity over existing infrastructure to a new infrastructure....

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