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In both 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. 

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 
192.168.0.123.” 

The following sequence shows the effects of the port forwarding rule you have defined:

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.

Your router receives the request message and looks in its rules table for any rules covering 
the handling of incoming port 80 traffic. Your port forwarding rule specifies that incoming port 
80 traffic should be forwarded to local IP address 192.168.0.123. Therefore, your router 
modifies the destination information in the request message:

The destination address is replaced with 192.168.0.123.

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

Your web server at 192.168.0.123 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.

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 can usually find this information by contacting the publisher of the application or the 
relevant user groups or news groups.