NAT66 exists, it just isn't a necessity in IPv6. There are also private IPv6 networks. They are called Unique Local Addresses (ULA) and are in the range fd00::

NAT66 exists, it just isn't a necessity in IPv6. There are also private IPv6 networks. They are called Unique Local Addresses (ULA) and are in the range fd00::/8. Which itself is so much better than RFC 1918 addresses. If you need private, non-Internet routable addresses, then you generate a random one. In the event two private networks need to communicate over VPN, for example, there is no clash.