Here's a link to using multiple IPs with one modem:
But I wouldn't bother, unless you know what you are doing.
Windows ICS (internet connection sharing) is not too bad. Go to "network connections", start->control panel->network connections. Right click on the connection that is connected to the internet. Choose properties. Go to the advanced tab and check the "Allow other network users to connect through this computer's internet connection" box.
You might have to have a hub/switch between the two computers. Or you'll need the right kind of ethernet cable - crossover cable.
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