IP tunnel

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.

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 Transport Layer Security, are removed.

IP tunneling often bypasses simple firewall rules transparently since the specific nature and addressing of the original datagrams are hidden.

A multicast-aware destination router would remove the LSRR option from the packet and restore the multicast IP address to the packet's IP destination field.