Port Forwarding allows you to select just the ports that you need opened, and forward traffic sent through those ports to the right computer. Typically this is used when you run a web server or a game (which only requires a few ports open).
The steps to use Port Forwarding are (including screenshots):
1. If you do not know your router's IP address, you can find out by opening a Command Prompt on your computer and typing in ipconfig. This will show you your computers' IP address (most likely 192.168.1.x or 192.168.2.x). Your router will be the IP address that is listed under "Default Gateway" (most likely 192.168.1.1 or 192.168.2.1).
2. Open a web browser and type in the IP address of your router. You will be presented with a Login screen. If you have not changed your Administrator password, then the username will be blank, and the password will be admin. It is highly recommended that you change this password. When you've logged in, you will be presented with the Basic Setup screen.
3. Click on the Application and Gaming Tab. This should bring up the Port Forwarding sub-tab.
4. Enter a name in the Application field to represent why you are forwarding the port. In the port fields, you can enter the same number in both (for forwarding one port) or the starting and ending numbers (for forwarding a range of ports). In the Protocol Field, you can choose either TCP, UDP, or Both (depending on the requirements of your application). In the IP address, you will enter the IP address of the computer which should receive the traffic. Finally, click the box next to the IP address to enable the forwarding. You will have to click Save Changes at the bottom of the screen.