Netgear AC1200 Owner's Manual

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 having observed the destination port number of 6667, 
your router creates an additional 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.

Upon receiving the incoming message to destination port 33333, your router checks its 
session table for an active session 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.

Upon receiving the incoming message to destination port 113, your router 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.

To configure port triggering, you need to know which inbound ports the application needs. 

Also, you need to know the number of the outbound port that will trigger the opening of the 

inbound ports. You can usually determine this information by contacting the publisher of the 
application or the relevant user groups or news groups.

Only one computer at a time can use the triggered application.

In both of the preceding examples, your computer initiates an application session with a 
server computer on the Internet. However, you might need to allow a client computer on the 
Internet to initiate a connection to a server computer on your network. Normally, your router 
ignores any inbound traffic that is not a response to your own outbound traffic. You can 
configure exceptions to this default rule by using the port forwarding feature. 

A typical application of port forwarding can be shown by reversing the client-server 

relationship from the previous web server example. In this case, a remote computer’s 
browser needs to access a web server running on a computer in your local network. Using 
port forwarding, you can tell the router, “When you receive incoming traffic on port 80 (the 
standard port number for a web server process), forward it to the local computer at