What is Port Triggering?

When local network devices are behind a NAT router, port forwarding is generally used to reach one local computer, as you need to specify the target IP address (or hostname).

Port triggering is a dynamic extension to port forwarding. It can open ports temporarily and close them when not in use.
With port triggering, the NAT router monitors traffic to the internet from outbound "trigger" port(s). When it finds such traffic, the router remembers the IP address of the local computer, and temporarily opens the "target" incoming port(s) to that same local IP.

Port triggering can be used to open the same ports/ranges to reach multiple local computers (not at the same time), as ports are dynamically closed when not in use.

For example, let's say an application requires the following port triggering rule:
Trigger ports: 8000-8010
Target ports: 9000-9010

In this example, the NAT router would listen for outbound traffic on the "trigger" ports 8000-8010. When/if it finds any, it would remember the local IP address that initiated the traffic. It would then temporarily forward incoming traffic on the "target" 9000-9010 ports to that same local IP address.