From what I understand, you have a Cable/DSL modem, and there is a PC plugged into it. If I am understanding you correctly, then the solution is to get a router. The router acts as a hub and it also distributes the internet, and internal IP addresses. You can send information through the router to other computers in your house as well.
Basically instead of this:
Make it like this:
And any router will do, really, but I would recommend a Linksys. I've been using my Linksys for years without a hitch, and it has all the features I'd need. Here's a good one that's not too expensive: