Carrier Grade NAT (CGNAT or CGN) is a concept that refers to the usage of NAT techniques to share public IPv4 addresses amongst users but on a much larger scale than with conventional NAT. An important reason behind the need for such large scale sharing is the exhaustion of available public IPv4 addresses.

CGNAT IPv4 address sharing is often similar to the usage of standard NAT but with the key difference that the sharing is moved back one step to the service provider level and closer to the public Internet. An ISP, for example, might then allocate private (RFC 1918) IPv4 addresses to connecting hosts instead of public IPv4 addresses and then NAT these flows through the limited number of public IPv4 addresses available to them. This results in an increased number of hosts sharing less IPv4 addresses.

CGNAT is also sometimes referred to as Large Scale NAT (LSN) and can also be referred to with the names Service Provider NAT44 or NAT444 (alternatively NAT44(4)).

cOS Stream provide tools to implement the CGNAT concept with the following features:

These features, both singly and in combinations, allow the administrator to provide both the interworking of IPv4 and IPv6 networks as well as the provision of a large scale NAT solution.