on networks N300R, User Manual

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N300 WiFi Router (N300R) User Manual
5-4 Fine-Tuning Your Network
v1.0, May 2012
“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, beginning 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 router.
3. Your router creates an entry in its internal session table describing this communication session
between your computer and the IRC server. Your 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 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 router using the NAT-assigned source port (as
in the previous example, let’s say port 33333) as the destination port. The IRC server also
sends an “identify” message to your router with destination port 113.
6. Upon receiving the incoming message to destination port 33333, your router checks its session
table to determine whether there is an active session for port number 33333. Finding an active
session, the 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 router checks its session
table and learns that there is an active session for port 113, associated with your computer. The
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 router eventually senses a period of inactivity in the
communications. The 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 will trigger 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.