This is some quite technical information, but it may help you to understand the foundations of everyday networking. Matt Baxter has some drawings that will show you how the structure of the headers for IP, TCP, UDP and ICMP looks byte-by-byte. It is just the normal information from the RFCs presented in an easy to read format.
RFCs are "Request for Comments", papers that are published to become Internet Standards. For example the RFC for the Internet Protocol IP is RFC 791. The information is more or less and text-only and it is kind of hard to read.