Port Triggering To Open Incoming Ports - NETGEAR DGND4000 User Manual

N750 Wireless Dual Band Gigabit ADSL Modem Router DGND4000
modem router stores the original information and then modifies the source information in the
request message, performing Network Address Translation (NAT):
• The source address is replaced with your wireless modem router's public IP address.
This requirement 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 wireless modem
router, such as 33333. This requirement is necessary because two computers could
independently be using the same session number.
Your wireless modem 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 wireless modem 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 wireless modem router.
Destination port number. 33333.
5. Upon receiving the incoming message, your wireless modem router checks its session
table to determine whether there is an active session for port number 33333. Finding an
active session, the wireless modem router then modifies the message to restore the original
address information replaced by NAT. Your wireless modem 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 wireless modem router eventually detects a
period of inactivity in the communications. Your wireless modem 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 wireless modem
router from a particular service port number, and replies from the remote computer to your
wireless modem router are directed to that port number. If the remote server sends a reply to
a different port number, your wireless modem 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 wireless modem router, you can tell
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