This section presents the following topics:
The IP-in-IP encapsulation protocol is used to tunnel between networks that have different capabilities or policies. For example, an IP-in-IP tunnel can be used to forward a multicast packet across a section of a network (such as an IPsec tunnel) that does not support multicast routing. An IP-in-IP tunnel can also be used to influence the routing of a packet, or to deliver a packet to a mobile device that uses Mobile IP.
In IP-in-IP encapsulation, a second IP header is inserted in front of the IP header of the original packet (the “passenger” packet). The new IP header has as source and destination addresses the addresses of the tunnel endpoints. The IP header of the payload packet identifies the original sender and receiver. When the encapsulated packet exits the tunnel, the outer IP header is stripped off, and the original IP packet is delivered to the final destination.