A variety of different tunneling techniques already exist for the purpose of interconnecting IPv6 networks using the infrastructure provided by existing IPv4 networks. For example, imagine if a company were to deploy IPv6 in two of their offices. Using various tunneling methods, the company could interconnect these offices over an IPv4-based network such as an existing WAN connection or the Internet. In order to do this, the routers providing the interconnection between the IPv6 and IPv4 networks must be dual stack devices. The advantages of using tunneling include the fact that it allows service providers to begin providing end-to-end IPv6 connections, even before they have fully converted their infrastructure to supporting IPv6.
A number of different methods for tunneling IPv6 over IPv4 networks are currently in use including manually configured tunnels, generic routing encapsulation (GRE) tunnels, automatic IPv6 to IPv4 tunnels that use techniques similar to IPv4-mapped IPv6 addresses, and more. One inherent limitation of IPv6 tunneling techniques is that they do not support common Internet connection methods like network address translation (NAT).
