This topic provides an introduction to a few transition mechanisms that can be used when migrating to an IPv6 network.

The key to successful adoption and deployment of IPv6 is the transition from the installed IPv4 base. The goal of all transition strategies is to facilitate the partial and incremental upgrade of hosts, servers, routers, and network infrastructure. There are many possible approaches, and some of the more likely approaches are described below. The transition strategy a company chooses to take varies based on the particular needs of that company.

Several migration issues must be addressed when the backbone routing protocol is IPv4. First, a mechanism is needed to allow communication between islands of IPv6 networks that are interconnected only using the IPv4 backbone. Tunneling of IPv6 packets over the IPv4 network can be used to connect the clouds. Second, end-to-end communication between IPv4 and IPv6 applications must be enabled. Several approaches to accomplish this exist; Application Layer Gateways, NAT-PT, and Bump-in-the-Stack are all possibilities. During the migration phase, it is likely that a combination of one, multiple, or all of these transition mechanisms can be used.

Application Layer Gateways (ALGs) enable IPv6-only applications to communicate with an IPv4-only peer. Using an ALG, the client connects to the ALG by using its native protocol (IPv4 or IPv6) and the ALG connects to the server by using the other protocol (IPv6 or IPv4).