NETGEAR DGND4000 User Manual page 113

N750 Wireless Dual Band Gigabit ADSL Modem Router DGND4000
the wireless modem router to open additional 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 wireless 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. 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 wireless modem router.
3. Your wireless modem router creates an entry in its internal session table describing this
communication session between your computer and the IRC server. Your wireless modem
router 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 wireless modem router creates an additional session entry to send any incoming port
113 traffic to your computer.
5. The IRC server sends a return message to your wireless modem router using the
NAT-assigned source port (as in the previous example, say port 33333) as the destination
port. The IRC server also sends an identify message to your wireless modem router with
destination port 113.
6. Upon receiving the incoming message to destination port 33333, 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 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 wireless modem router
checks its session table and learns that there is an active session for port 113 associated
with your computer. The wireless 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 wireless modem router eventually senses a period
of inactivity in the communications. The wireless modem router then removes the session
information from its session table, and incoming traffic is no longer accepted on port
numbers 33333 or 113.
To configure port triggering, you need to know which inbound ports the application needs.
Also, you need to know the number of the outbound port that triggers the opening of the
inbound ports. You can usually determine this information by contacting the publisher of the
application or user groups or newsgroups.
Note: Only one computer at a time can use the triggered application.
Advanced Settings
113