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N150 Wireless ADSL2+ Modem Router DGN1000Bv3
stores the original information, performs Network Address Translation (NAT) on the source
address and port, and sends this request message through the Internet to the IRC server.
4. Noting your port triggering rule and having observed the destination port number of 6667,
your modem router creates an additional session entry to send any incoming port 113 traf
fic
to your computer.
5. The IRC server sends a return message to your modem router using the NAT
-assigned
source port (for example, port 33333) as the destination port. The IRC server also sends an
“identify” message to your modem router with destination port 113.
6. Upon receiving the incoming message to destination port 33333, 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 restores the original address
information replaced by NAT and sends this reply message to your computer
.
7. Upon receiving the incoming message to destination port 113, your modem router checks
its session table and learns that there is an active session for port 1
13 associated with your
computer. The modem router replaces the message’s destination IP address with your
computer’s IP address and forwards the message to your computer.
8. When you finish your chat session, your modem router eventually senses a period of
inactivity in the communications. The modem router then removes the session information
from its session table, and incoming traf
fic is no longer accepted on port numbers 33333 or
113.
To configure port triggering, you must know which inbound ports the application needs. Also,
you must know the number of the outbound port that will trigger the opening of the inbound
ports. You can usually determine this information by contacting the publisher of the
application or the user groups or news groups.
Note: Only one computer at a time can use the triggered application.
Port Forwarding to Permit External Host Communications
In both of the preceding examples, your computer initiates an application session with a
server computer on the Internet. However, you might need to allow a client computer on the
Internet to initiate a connection to a server computer on your network. Normally, your modem
router ignores any inbound traffic that is not a response to your own outbound traffic. You can
configure exceptions to this default rule by using the port forwarding feature.
A typical application of port forwarding can be shown by reversing the client-server
relationship from the previous web server example. In this case, a remote computer
’
s
browser needs to access a web server running on a computer in your local network. Using
port forwarding, you can tell the modem router, “When you receive incoming traffic on port 80
(the standard port number for a web server process), forward it to the local computer at
192.168.0.123.”