IPv6 Transition Method and Strategies

One nice feature of moving your network to IPv6 is that you don't have to do it all in one step. Various migration strategies support both IPv4 and IPv6 as you migrate from the former to the latter.

Most common method for transition is given in following table.

Transition Method	Description
Dual stacking	Devices such as PCs and routers run both IPv4 and IPv6, and thus have two sets of addresses.
Manual IPv6-over-IPv4 (6to4) tunneling	IPv6 packets are tunneled across an IPv4 network by encapsulating them in IPv4 packets. This requires routers configured with dual stacks.
Dynamic 6to4 tunneling	Allows IPv6 localities to connect to other IPv6 localities across an IPv4 backbone, such as the Internet, automatically. This method applies a unique IPv6 prefix to each locality without having to retrieve IPv6 addressing information from address registries or ISPs.
Intra-Site Automatic Tunnel Addressing Protocol (ISATAP) tunneling	Uses virtual links to connect IPv6 localities together within a site that is primarily using IPv4. Boundary routers between the two addressing types must be configured with dual stacks.
Teredo tunneling	Instead of using routers to tunnel packets, Teredo tunneling has the hosts perform the tunneling. This requires the hosts to be configured with dual stacks. It is commonly used to move packets through an IPv4 address translation device.
NAT Proxying and Translation (NAT-PT)	Has an address translation device translate addresses between an IPv6 and IPv4 network and vice versa.

Dual Stacking

In dual stacking, a device runs both protocol stacks: IPv4 and IPv6. Of all the transition methods, this is the most common one.

Dual stacking can be accomplished on the same interface or different interfaces of the device. Figure shows an example of dual stacking on a router, where Network A has a mixture of devices configured for the two different protocols, and the router configured in a dual stack mode. Older IPv4-only applications can still work while they are migrated to IPv6 by supporting newer APIs to handle IPv6 addresses and DNS lookups with IPv6 addresses. 

The main disadvantage of dual stacking on a segment is that devices configured using only one stack must forward their traffic to a dual-stacked device, such as a router, which must then forward the traffic back to the same segment using the other stack.

This is an inefficient use of bandwidth, but it does allow devices using both protocol stacks to coexist on the same network segment.