How Port Triggering Changes The Communication Process - NETGEAR WNR612v2 - Wireless-N 150 Router User Manual

Wireless-N 150 Router WNR612v2 User Manual
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 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

Usually, during an Internet session your computer sends outbound requests. The router
sends replies to same port number where the session is. Replies to a different port number,
are discarded. However, some application servers (such as FTP and IRC servers) send
replies to multiple port numbers. With port triggering, you can tell the router to open more
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:
You open an IRC client program, beginning a chat session on your computer.
1.
Your IRC client composes a request message to an IRC server using a destination port
2.
number of 6667, the standard port number for an IRC server process. Your computer then
sends this request message to your router.
Chapter 6: Fine-Tuning Your Network | 55