An IP tunnel is an
Internet Protocol
The Internet Protocol (IP) is the network layer communications protocol in the Internet protocol suite for relaying datagrams across network boundaries. Its routing function enables internetworking, and essentially establishes the Internet.
...
(IP) network communications channel between two networks. It is used to transport another network protocol by
encapsulation of its
packets.
IP tunnels are often used for connecting two disjoint IP networks that don't have a native routing path to each other, via an underlying routable protocol across an intermediate transport network. In conjunction with the
IPsec
In computing, Internet Protocol Security (IPsec) is a secure network protocol suite that authenticates and encrypts packets of data to provide secure encrypted communication between two computers over an Internet Protocol network. It is used in ...
protocol they may be used to create a
virtual private network between two or more private networks across a public network such as the
Internet
The Internet (or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a ''internetworking, network of networks'' that consists ...
. Another prominent use is to connect islands of
IPv6
Internet Protocol version 6 (IPv6) is the most recent version of the Internet Protocol (IP), the communications protocol that provides an identification and location system for computers on networks and routes traffic across the Internet. I ...
installations across the
IPv4
Internet Protocol version 4 (IPv4) is the fourth version of the Internet Protocol (IP). It is one of the core protocols of standards-based internetworking methods in the Internet and other packet-switched networks. IPv4 was the first version d ...
Internet.
In IP tunnelling, every IP packet, including addressing information of its source and destination IP networks, is encapsulated within another packet format native to the transit network.
At the borders between the source network and the transit network, as well as the transit network and the destination network, gateways are used that establish the end-points of the IP tunnel across the transit network. Thus, the IP tunnel endpoints become native IP routers that establish a standard IP route between the source and destination networks. Packets traversing these end-points from the transit network are stripped from their transit frame format headers and trailers used in the
tunnelling protocol and thus converted into native IP format and injected into the IP stack of the tunnel endpoints. In addition, any other protocol encapsulations used during transit, such as IPsec or