User Manual

13.
Click the Apply button.
The service is now in the Portmap table. You must enable port triggering before the
cable modem router used port triggering for the service that you added. See the
following section.
Application Example: Port Triggering for Internet Relay
Chat
Some application servers, such as FTP and IRC servers, send replies to multiple port
numbers. Using port triggering, you can tell the cable modem router to open more
incoming ports when a particular outgoing port starts 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 cable modem router, “When you initiate a session with destination port
6667, you must also allow incoming traffic on port 113 to reach the originating
computer.” The following sequence shows the effects of this port triggering rule:
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 cable modem router.
3. Your cable modem router creates an entry in its internal session table describing
this communication session between your computer and the IRC server. Your cable
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.
Based on your port triggering rule and the destination port number of 6667, your
cable modem router creates another session entry to send any incoming port 113
traffic to your computer.
5. The IRC server sends a return message to your cable 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 cable modem router with
destination port 113.
6. When your cable modem router receives the incoming message to destination port
33333, it checks its session table to see if a session is active for port number 33333.
Finding an active session, the cable modem router restores the original address
information replaced by NAT and sends this reply message to your computer.
7. When your cable modem router receives the incoming message to destination port
113, it checks its session table and finds an active session for port 113 associated
with your computer. The cable modem router replaces the message’s destination
User Manual97Specify Internet Port Settings
AC1750 WiFi Cable Modem Router