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N300 WiFi Router (N300R) User Manual
Fine-Tuning Your Network 5-5
May 2012
How Port Forwarding Changes the Communication Process
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.
A typical application of port forwarding can be shown by reversing the client-server relationship
from our 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 Internet Explorer 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:
• The destination address is the IP address of www.example.com, which is the address of
your router.
• The destination port number is 80, 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 user
groups or newsgroups.