PAT

PAT (Port Address Translation), commonly called NAT overload, is the variant of NAT that allows many internal hosts to share a single public IPv4 address by rewriting the source port of each outbound flow. The router keeps a stateful translation table that maps every internal IP + source port pair to a unique external port on the public IP. Return traffic is demultiplexed back to the right internal host by looking up the external port in the same table.

In practice PAT is what almost every home and small-office router actually does, even when the configuration UI calls it "NAT". Hundreds of devices behind one subscriber line can reach the internet through a single public IPv4 because each TCP/UDP flow gets its own ephemeral external port (typically from the 1024-65535 range). The trade-off is asymmetric reachability: outbound flows work transparently, but inbound connections fail unless an explicit port forward, UPnP/NAT-PMP mapping, or hole-punching mechanism creates a static entry in the translation table. The same model is reused at carrier scale as CGN/CGNAT, where many subscribers share an even smaller pool of public addresses.