A mapped port is a port number that a NAT device translates or forwards from a public-facing address to an internal host and service. It creates a reachable path through the gateway, so traffic sent to the external port is delivered to the private system behind it.
Mapped ports matter because they turn translation state into exposure. If a router is misconfigured, overloaded, or manipulated, an attacker may discover or redirect these mappings and reach services that were meant to stay internal. Defenders track mapped ports to understand what is reachable from the internet, limit unnecessary forwarding, and detect unexpected changes in NAT tables or connection state.