How Port Triggering Changes The Communication Process - NETGEAR WNR612v2 User Manual

The source address is replaced with your router's public IP address.
•
This is necessary because your computer uses a private IP address that is not
globally unique and cannot be used on the Internet.
The source port number is changed to a number chosen by the router, such as 33333.
•
This is necessary because two computers could independently be using the same
session number.
Your router then sends this request message through the Internet to the Web server at
www.example.com.
The Web server at www.example.com composes a return message with the requested Web
4.
page data. The return message 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 the public IP address of your router.
•
The destination port number is 33333.
•
The Web server then sends this reply message to your router.
Upon receiving the incoming message, your router checks its session table to determine
5.
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.
When you finish your browser session, your router eventually senses a period of inactivity in
6.
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.
58 | Chapter 6: Fine-Tuning Your Network
Wireless-N 150 Router WNR612v2 User Manual