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N300 Wireless Router WNR2000v4
Your router then sends this request message through the Internet to the web server at
www.example.com.
4. The web server at www.example.com composes a return message with the requested web
page data. The return message contains the following address and port information. The
web server then sends this reply message to your router.
• Source address. The IP address of www.example.com.
• Source port number. 80, which is the standard port number for a web server
process.
• Destination address. The public IP address of your router.
• Destination port number. 33333.
5. Upon receiving the incoming message, your router checks its session table to determine
whether there is an active session for port number 33333. Finding an active session, the
router then modifies the message to restore the original address information replaced by
NAT. Your router sends this reply message to your computer, which displays the web
page from www.example.com. The message now contains the following address and port
information.
• Source address. The IP address of www.example.com.
• Source port number. 80, which is the standard port number for a web server
process.
• Destination address. Your computer’s IP address.
• Destination port number. 5678, which is the browser session that made the initial
request.
6. When you finish your browser session, your router eventually detects a period of inactivity in
the communications. Your router then removes the session information from its session
table, and incoming traffic is no longer accepted on port number 33333.
Port Triggering to Open Incoming Ports
In the preceding example, requests are sent to a remote computer by your router from a
particular service port number, and replies from the remote computer to your router are
directed to that port number. If the remote server sends a reply back to a different port
number, your router does not recognize it and discards it. However, some application servers
(such as FTP and IRC servers) send replies back to multiple port numbers. Using the port
triggering function of your 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.”