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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 this port triggering rule:
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 observing the destination port number of 6667, your router creates another
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 (for example, port
33333) as the destination port and sends an “identify” message to your router with destination port 113.
6. 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.
7. 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.
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.
Customize Internet Traffic Rules for Ports
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