Port Triggering To Open Incoming Ports - NETGEAR N900 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.
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 if
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 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 to multiple port numbers. Using the port triggering
function of your router, 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
N900 Wireless Dual Band Gigabit Router R4500
Advanced Settings
90