For the rapid exhaustion of IPv4 address pool against the slow development of IPv6, IPv4/IPv6 co-existence/transition would be a long period. In the IPv4/IPv6 transition process, there are many NAT- like technologies active in the internet. However, the NAT boxes such as IPv4 NAT, IPv4/IPv6 NAT are so poor in their reliability and scalability, which put a severe threat on the development of IPv4/IPv6 transition. This document defines a reliable and scalable NAT (RS- NAT) mechanism to solve the problem.