Reading the DNS, TCP/IP, and HTTP RFCs would give you the authoritative information on the specs. Depending on whether you like this kind of stuff, they're either long and a bit dry, or really interesting :-) Also, sometimes it's difficult to figure out which RFCs are the core ones and which ones are just extensions that don't get much use. So perhaps Wikipedia is a slightly gentler start: Domain Name System, Internet protocol suite, and Hypertext Transfer Protocol. Also, it will help you a great deal if at the same time you learn how to use Wireshark; reading the specs will help you interpret the traces and playing with real-life traces should help show the practical application of what you're reading.