Depending on what you want to do on what O/S, batteries may or may not be included. I use Moonlight on a Mac to control a Windows 10 PC running Sunshine--lower latency than Remote Desktop (which I don't have anyway in the Home edition), and nicer looking than Parsec.
When you connect a patch cable directly (no crossover cable needed in the 21st century), you'll likely find that each system has a self-assigned IP in the 169.254 network.
A friend has reminded me that with Linux, it can be as basic as piping tar between the two machines using nc. (There's probably a friendly frontend to this somewhere that will allow the machines to mount each other's filesystems with fuse, though...)
When you connect a patch cable directly (no crossover cable needed in the 21st century), you'll likely find that each system has a self-assigned IP in the 169.254 network.