What kind of modem do you have? That will help to know
Have you ever been to the control panel of that modem by entering the default gateway in your browser?
that is where you can change the port settings.
Apple Mac OS X: any of the following:
* Pull down the Apple menu, select System Properties, click Network. In the pull-down Show: select the network interface in use. Click tab TCP/IP and look for the line Router.
* Open a Terminal window, type the command ipconfig getoption en0 router (where en0 is the name of the interface in use)