Linksys? Don't they have a psuedo-web HTML menu that you access by typing in the Router's ip address? I know my D-Link has something like that. Once you get to that menu you will have to option to open ports, you type in the port and select TCP or UDP and some other options and it should be done. It seemed relatively simple for mine, but yours may have an entirely different system. However that is as far as my knowledge goes because when it comes to learning this stuff, I have to take it <i>real</i> slow.
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Good luck though.
user= blank
password = admin
go to advanced at the top > then forwarding > put in your IP so that it forwards to your computer and enter the port range that you want.