Netgear AC1750 Owner's Manual

Some application servers, such as FTP and IRC servers, send replies to multiple port 
numbers. Using port triggering, you can tell the modem router to open more incoming ports 
when a particular outgoing port starts a session.

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 modem 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 this port triggering rule:

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 modem router.

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

communication session between your computer and the IRC server. Your modem 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 

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

computer.

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

source port (for example, port 33333) as the destination port and sends an “identify” 

message to your modem router with destination port 113.

When your modem 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 modem router restores the original address information replaced by NAT and 

sends this reply message to your computer.

When your modem 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 

modem 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 modem router eventually senses a period of 

inactivity in the communications and removes the session information from its session table, 

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