TCP/IP Builder is a Windows Socket testing tool. You will be able to create a socket manually, connect it (or listen for connections), send and receive data through it.
This source code is at least 15 years old. I strongly disencourage you from taking it as an example for anything you might be developing today. The tool works just fine and there is no risk in running it. But the code style, library usage and C with C++ mixing is awful.