There are a lot of posts in these forums asking about how to get started in ethical hacking. The most imperative place to start has to be a complete understanding of TCP/IP. Without you will not only be lost when reading the materials and experimenting with the tools, but the hows and whys of most attacks will go right over your head.

Well do I have the reference for you:

The TCP/IP Guide: A Comprehensive, Illustrated Internet Protocols Reference

This is THE definitive guide to the topic with not only 1500 pages of reference material but also, as the title suggests, illustrations of the concepts as well. With that much info, it clearly is not meant to be read from cover to cover. Also, those who have reviewed it say that it is great for the newbie as well as the pro.

I encourage you to read reviews to draw your own conclusions.

But the best part is that the author, Charles Kozierok, has published the entire book online. It's in HTML, contains ads (careful of the double lined links) and not as accessible as the book would be, but it's there AND FREE!

http://www.tcpipguide.com/free/index.htm

Let us know what you think,
Don