Netgear Orbi AC3000 Tri-band Wi-Fi® Satellite Owner's Manual

An example is Internet Relay Chat (IRC). Your computer connects to an IRC server at 

destination port 6667. The IRC server not only responds to your originating source port, but 
also sends an “identify” message to your computer on port 113. Using port triggering, you can 
tell the router, “When you initiate a session with destination port 6667, you must also allow 
incoming traffic on port 113 to reach the originating computer.” The following sequence 
shows the effects of the port triggering rule you define:

You open an IRC client program to start a chat session on your computer. 

Your IRC client composes a request message to an IRC server using a destination port 

number of 6667, the standard port number for an IRC server process. Your computer then 

sends this request message to your router.

Your router creates an entry in its internal session table describing this communication 

session between your computer and the IRC server. Your router stores the original 

information, performs Network Address Translation (NAT) on the source address and port, 

and sends this request message through the Internet to the IRC server. 

Noting your port triggering rule and observing the destination port number of 6667, your 

router creates another session entry to send any incoming port 113 traffic to your computer.

The IRC server sends a return message to your router using the NAT-assigned source port 

(for example, port 33333) as the destination port. The IRC server also sends an “identify” 

message to your router with destination port 113.

When your router receives the incoming message to destination port 33333, it checks its 

session table to see if a session is active for port number 33333. Finding an active session, 

the router restores the original address information replaced by NAT and sends this reply 

message to your computer.

When your router receives the incoming message to destination port 113, it checks its 

session table and finds an active session for port 113 associated with your computer. The 

router replaces the message’s destination IP address with your computer’s IP address and 

forwards the message to your computer.

When you finish your chat session, your router eventually senses a period of inactivity in the 

communications. The router then removes the session information from its session table, 

and incoming traffic is no longer accepted on port numbers 33333 or 113.