Chapter 5: Fine-Tuning Your Network | 59
N150 Wireless Router User Manual
The Web server then sends this reply message to your router.
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, restoring the original address information replaced by
NAT. The message now contains the following address and port information:
• The source address is the IP address of www.example.com.
• The source port number is 80, the standard port number for a Web server process.
• The destination address is your computer’s IP address.
• The destination port number is 5678, the browser session that made the initial
request.
Your router then sends this reply message to your computer, which displays the Web
page from www.example.com.
6. When you finish your browser session, your router eventually senses 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.
How Port Triggering Changes the Communication Process
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 will not recognize it and will discard 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 must 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, beginning 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.