TCP/IP Illustated Volume 1: The Protocols by W. R. Stevens is the classic textbook and a great place to start off. You can then read the IETF RFCs, which are us

TCP/IP Illustated Volume 1: The Protocols by W. R. Stevens is the classic textbook and a great place to start off. You can then read the IETF RFCs, which are usually well written and completely readable to anybody with a computing background.