That's Internet Connection Sharing. One machine in the network is connected to the Internet. Other machines connect to that machine (sort of proxy). It dates from the good old days of dial-up when there were no multi-PC routers, so it was a physical necessity (only one machine could be connected to the phone line).
ICS is still supported on XP. But it's somewhat clumsy to setup. I've once seen it working (setup by a professional) and read many, many posts here from people who couldn't.
Setup: I have an old laptop which is connected to the main router via wireless and my desktop via ethernet. I currently have the wireless network connection "shared" and so my desktop can connect to the internet. However, other computers on the network cannot access my desktop, only my laptop. This also means that no ports from the internet can be forwarded to my desktop, only my laptop.
<div><div>Is it possible for my laptop to act as a switch or a gateway? the laptop still being able to access internet is preferred but not required.