IP tunnels (also referred to simply as tunnels in this book) provide a means for transporting data packets between domains when the protocol in those domains is not supported by intermediary networks. For example, IPv6 networks require a way to communicate outside their borders in an environment where most networks use the IPv4 protocol. This communication is possible by using tunnels. IP tunnels provide a virtual link between two nodes that are reachable by using IP. The link can thus be used to transport IPv6 packets over the IPv4 networks to enable IPv6 communication between the two IPv6 sites.