Netgear WNR2500 User Manual - Page 88

N450 Wireless Router WNR2500 Port Triggering to Open Incoming Ports In the preceding example, your router sends requests to a remote computer from a particular service port number, and replies from the remote computer to your router are directed to that port number. If the remote server sends a reply to a different port number, your router does not recognize it and discards it. However, some application servers (such as FTP and IRC servers) send replies to multiple port numbers. Using the port triggering function of your router, you can tell the router to open more 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 router, "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 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. When your router receives the incoming message to destination port 33333, it checks its session table for 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. When your 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 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 must know which inbound ports the application needs. Also, you must know the number of the outbound port that triggers the opening of the inbound ports. You can usually find this information by contacting the publisher of the application or user groups or newsgroups. Advanced Settings 88