Carrier-Grade Network Address Translation (CGNAT) modifies the source port numbers of outgoing network packets as they traverse the network infrastructure of an Internet Service Provider (ISP). This mechanism allows multiple subscribers within a private network to share a single public IP address. For example, numerous households connected to the same ISP can access external websites seemingly from the same public IP, differentiating their traffic through unique source port assignments.
This approach is vital in mitigating IPv4 address exhaustion. By sharing a single public address among many users, ISPs can continue providing internet access without requiring a unique public IPv4 address for each subscriber. This practice has been particularly important since the late 2000s, as the pool of available IPv4 addresses has dwindled. This technology provides a transitional measure towards the widespread adoption of IPv6.
