If you want to learn about IP, TCP, UDP and some of the protocols below this I would highly recommend reading Richard Stevens book TCP/IP Illustrated, Volume 1: The Protocols.

For two reasons: It's probably one of the best introductions to the subject that has ever been written, and it's a model example of how a technical book should be written.

I'd be hard pressed to find a reason not to go this route at least once in your life. I know the material pretty well but I still re-read Stevens books every few years just because it is so good.