IPv6 NAT-PT is to be used with IPv4 to IPv6 migration scenarios and it's purpose is to provide bi-directional connectivity between IPv4 and IPv6 domains. Cisco points out that many other transition techniques are possible, and NAT-PT (Network Address Translation - Protocol Translation) should not be used when other, more "native" options exist, such as having dual stack hosts communicate directly through dual stack routers. Another example provided of when NAT-PT is not needed is when two...
Quite many people don't pay attention to the difference in handling packets on interfaces configured for NAT inside and outside. Here is an example to demonstrate how NAT "domains" interact with routing. Consider three routers connected in the following manner: