Port triggering is a configuration option on a NAT-enabled router that controls communication between internal and external host machines in an IP network. It is similar to port forwarding in that it enables incoming traffic to be forwarded to a specific internal host machine, although the forwarded port is not open permanently and the target internal host machine is chosen dynamically.
Description
When two networks communicate through a NAT-router, the host machines on the internal network behave as if they have the IP address of the NAT-router from the perspective of the host machines on the external network. Without any traffic forwarding rules, it is impossible for a host machine on an external network (host B) to open a connection to a host machine in the internal network (host A). This is because the connection can only be targeted to the IP of the NAT-router, since the internal network is hidden behind NAT. With port triggering, when some host A opens a connection to a host B using a predefined port or ports, then all incoming traffic that the router receives on some predefined port or ports is forwarded to host A. This is the 'triggering' event for the forwarding rule. The forwarding rule is disabled after a period of inactivity.