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N300 Wireless Router WNR2000v4
Port Forwarding and Port Triggering Configuration
Concepts
By default, the router blocks inbound traffic from the Internet to your computers except replies
to your outbound traffic. You might need to create exceptions to this rule for these purposes:
• To allow remote computers on the Internet to access a server on your local network.
• To allow certain applications and games to work correctly when their replies are not
recognized by your router.
Your router provides two features for creating these exceptions: port forwarding and port
triggering. The next sections provide background information to help you understand how
port forwarding and port triggering work, and the differences between the two.
Remote Computer Access Basics
When a computer on your network needs to access a computer on the Internet, your
computer sends your router a message containing the source and destination address and
process information. Before forwarding your message to the remote computer, your router
has to modify the source information and create and track the communication session so that
replies can be routed back to your computer.
Here is an example of normal outbound traffic and the resulting inbound responses:
1. You open a browser, and your operating system assigns port number 5678 to this
browser session.
2. You type http://www.example.com into the URL field, and your computer creates a web page
request message with the following address and port information. The request message is
sent to your router.
• Source address. Your computer’s IP address.
• Source port number. 5678, which is the browser session.
• Destination address. The IP address of www.example.com, which your computer
finds by asking a DNS server.
• Destination port number. 80, which is the standard port number for a web server
process.
3. Your router creates an entry in its internal session table describing this communication
session between your computer and the web server at www.example.com. Before sending
the web page request message to www.example.com, your router stores the original
information and then modifies the source information in the request message, performing
Network Address Translation (NAT):
• The source address is replaced with your router’s public IP address. This is
necessary because your computer uses a private IP address that is not globally
unique and cannot be used on the Internet.
• The source port number is changed to a number chosen by the router, such as
33333. This is necessary because two computers could independently be using the
same session number.