How Port Forwarding Differs from Port Triggering

N900 Wireless Dual Band Gigabit Router WNDR4500

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.

A typical application of port forwarding can be shown by reversing the client-server relationship from the previous web server example. In this case, a remote computer’s browser needs to access a web server running on a computer in your local network. Using port forwarding, you can tell the router, “When you receive incoming traffic on port 80 (the standard port number for a web server process), forward it to the local computer at 192.168.1.123.” The following sequence shows the effects of the port forwarding rule you have defined:

1.The user of a remote computer opens a browser and requests a web page from www.example.com, which resolves to the public IP address of your router. The remote computer composes a web page request message with the following destination information:

Destination address. The IP address of www.example.com, which is the address of your router.

Destination port number. 80, which is the standard port number for a web server process.

The remote computer then sends this request message through the Internet to your router.

2.Your router receives the request message and looks in its rules table for any rules covering the disposition of incoming port 80 traffic. Your port forwarding rule specifies that incoming port 80 traffic should be forwarded to local IP address 192.168.1.123. Therefore, your router modifies the destination information in the request message:

The destination address is replaced with 192.168.1.123.

Your router then sends this request message to your local network.

3.Your web server at 192.168.1.123 receives the request and composes a return message with the requested web page data. Your web server then sends this reply message to your router.

4.Your router performs Network Address Translation (NAT) on the source IP address, and sends this request message through the Internet to the remote computer, which displays the web page from www.example.com.

To configure port forwarding, you need to know which inbound ports the application needs. You usually can determine this information by contacting the publisher of the application or the relevant user groups and newsgroups.