This document describes one of the network models that are designed
for smooth transition to IPv6. It is called NAT444 model. NAT444
model is composed of IPv6, and IPv4 with Carrier Grade (CGN).
NAT444 is the only scheme not to require replacing Customer Premises
Equipment (CPE) even if IPv4 address exhausted. But it must be noted
that NAT444 has serious restrictions i.e. it limits the number of
sessions per CPE so that rich applications such as AJAX and RSS feed
cannot work well.
Therefore, IPv6 which is free from such a difficulty has to be
introduced into the network at the same time. In other words, NAT444
is just a tool to make IPv6 transition easy to be swallowed. It is
designed for the days IPv4 and IPv6 co-existence.