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CENTRIA WNDR4700/WNDR4720
router, you can tell the router to open additional 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 router, “When you initiate a session with destination port 6667, you have to also allow
incoming traffic 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:
1. You open an IRC client program to start a chat session on your computer.
2. 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.
3. 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.
4. 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.
5. The IRC server sends a return message to your 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 router with destination port 113.
6. Upon receiving the incoming message to destination port 33333, your router checks its
session table to determine whether there is 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.
7. Upon receiving the incoming message to destination port 113, your router checks its
session table and learns that there is 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.
8. 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. Usually, you can determine this information by contacting the publisher of the
application or the relevant user groups or news groups.
Note: Only one computer at a time can use the triggered application.