Advanced Settings
108
N150 Wireless ADSL2+ Modem Router DGN1000Bv3
Your 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 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 modem router
•Destination port number. 33333
5. Upon receiving the incoming message, your modem router checks its session table to
determine whether there is an active session for port number 33333. Finding an active
session, the modem router then modifies the message to restore the original address
information replaced by NAT. Your 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 modem router eventually detects a period of
inactivity in the communications. Your 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
Some application servers (such as FTP and IRC servers) send replies to multiple port
numbers. Using the port triggering function of your modem router, you can tell the modem
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 modem 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 to start 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 modem router.
3. Your modem router creates an entry in its internal session table describing this
communication session between your computer and the IRC server. Your modem router