Carrier Grade Network Address Translation (CGNAT) is a type of network address translation (NAT) used by Internet Service Providers (ISPs) in IPv4 network design. Unlike standard NAT, CGNAT adds an additional translation layer, allowing ISPs to translate private IPv4 addresses to public IPv4 addresses. This enables the sharing of small pools of public addresses among many end users, particularly in residential networks. CGNAT helps mitigate IPv4 address exhaustion by permitting the reuse of public IP addresses across multiple customers.
CGNAT assigns a single public IP address to multiple customer routers, sharing the IP address among them. It is commonly used in mobile phone networks and is increasingly being adopted in fixed connectivity scenarios. However, CGNAT can have limitations such as preventing customers from using port forwarding due to the way NAT is implemented in carrier-grade networks.
Cisco describes Carrier Grade Network Address Translation (CGN) as a large-scale NAT that translates private IPv4 addresses into public IPv4 addresses, supporting the preservation of public IPv4 addresses by processing subscriber traffic through private IPv4 networks.