Netgear 13300241 User Manual

servers (such as FTP and IRC servers) send replies to multiple port numbers. Using the port 

triggering function of your modem router, you can tell the modem router to open more 

incoming ports when a particular outgoing port originates 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 

allow incoming traffic also on port 113 to reach the originating computer.” Using steps similar 

to the preceding example, the following sequence shows the effects of the port triggering rule 

you have defined:

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 having observed the destination port number of 6667, 

your modem 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  modem router using the NAT-assigned 

source port (as in the previous example, say port 33333) as the destination port.  The IRC 

server also sends an identify message to your  modem router with destination port 113.

Upon receiving the incoming message to destination port 33333, your  modem router 

checks its session table to determine whether there is an active session for port number 

33333. Finding an active session, the  modem router restores the original address 

information that is replaced by NA T and sends this reply message to your computer .

Upon receiving the incoming message to destination port 1 13, your modem router checks 

its session table and learns that there is an active session for port 1 13, associated with your 

computer. The modem router replaces the destination IP address of the message with the 

IP address of your computer 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.  The modem router then removes the session information 

from its session table, and incoming traf fic is no longer accepted on ports 33333 or 1 13.

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 triggers the opening of the 

inbound ports. You can usually determine this information by contacting the publisher of the 

application or user groups or newsgroups.

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