19216811.liveAdvanced Settings
91
N900 Wireless Dual Band Gigabit Router R4500
tell the 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 those in
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 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, 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 if 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 should 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.
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 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.