Network address translation

IPv4 NAT translates one IPv4 (private) address into another IPv4 (external) address. IPv6 NAT-PT translates an IPv4 address into an IPv6 address.

Rules: There are several limitations with NAT-PT:

All requests and responses pertaining to a session must be routed through the same NAT-PT translator.
There is a protocol translation limitation because a number of IPv4 fields have changed meaning in IPv6. Details of IPv4 to IPv6 protocol translation can be found in the Stateless IP/ICMP Translation Algorithm (SIIT) RFC.
If an application carries the IP address in the payload, ALGs must be incorporated.
Lack of end-to-end security. The two end nodes that seek IPSec network level security must both use IPv4 or IPv6.
DNS messages and DNSSEC translation. An IPv4 end-node that demands DNS replies be signed rejects replies that have been tampered with by NAT-PT.