Carrier-grade NAT

Carrier-grade NAT (CGN or CGNAT) (also known as large-scale NAT, LSN) is a type of network address translation (NAT) used by Internet service providers (ISPs) in IPv4 network design. With CGN, end sites, in particular residential networks, are configured with private network addresses that are translated to public IPv4 addresses by middlebox network address translator devices embedded in the network operator's network, permitting the sharing of small pools of public addresses among many end users. This essentially repeats the traditional customer-premises NAT function at the ISP level.

Carrier-grade NAT is often used for mitigating IPv4 address exhaustion.

One use scenario of CGN has been labeled as NAT444, because some customer connections to Internet services on the public Internet would pass through three different IPv4 addressing domains: the customer's own private network, the carrier's private network and the public Internet.

Another CGN scenario is Dual-Stack Lite, in which the carrier's network uses IPv6 and thus only two IPv4 addressing domains are needed. CGN may also be applied using ISP-level NAT64 translators in combination with 464XLAT, which eliminates IPv4 entirely at the residential network.

CGN techniques were first used in 2000 to accommodate the immediate need for large numbers of IPv4 addresses in General Packet Radio Service (GPRS) deployments of mobile networks. Estimated CGN deployments increased from 1,200 in 2014 to 3,400 in 2016, with 28.85% of the studied deployments appearing to be in mobile operator networks.

Shared address space

If an ISP deploys a CGN, and uses RFC 1918 address space to number customer gateways, the risk of address collision, and therefore routing failures, arises when the customer network already uses an RFC 1918 address space.