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D6200 WiFi DSL Modem Router
• Destination address. The public IP address of your WiFi DSL modem router.
• Destinati
on port number. 33333.
5. Upon receiving the
incoming message, your WiFi DSL modem router checks its session
table to determine whether an active session for port number 33333 exists. Finding an active
session, the WiFi DSL modem router then modifies the message to restore the original
address information replaced by NAT. Your WiFi DSL 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 add
ress. The IP address of www.example.com.
• Source port nu
mber. 80, which is the standard port number for a web server
process.
• Destinati
on address. Your computer’s IP address.
• Destinati
on port number. 5678, which is the browser session that made the initial
request.
6. Wh
en you finish your browser session, your WiFi DSL modem router eventually detects a
period of inactivity in the communications. Your WiFi DSL 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 WiFi DSL modem
router from a particular service port number, and replies from the remote computer to your
WiFi DSL modem router are directed to that port number. If the remote server sends a reply
to a different port number, your WiFi DSL 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 WiFi DSL modem router, you can tell
the WiFi DSL modem router to open more incoming ports when a particular outgoing port
originates a session.
An example is Internet Relay Chat (IRC). You
r 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 WiFi DSL modem router, “When you initiate a session with destination port 6667, you
have to 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. Y
ou open an IRC client program to start a chat session on your computer.
2. Y
our 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 WiFi DSL modem router.
3. Y
our WiFi DSL modem router creates an entry in its internal session table describing this
communication session between your computer and the IRC server. Your WiFi DSL modem
router stores the original information, performs Network Address Translation (NAT) on the