Security - How Can you Learn Internet Self-Defense?

notacracker asks: "A friend and I are trying to learn about network security. I figure it would be more fun if we set up a two machine local area network, and practiced breaking in and detecting break-ins. But where to start ? It's easy enough to find a cookbook (eg O'Reilly) on security, but where is the equivalent to an O'Reilly book on cracking and actively defending a system? It sounds like someone has been toying with this idea over at ZDNet as well. You might want to check out their free-for-all hackfest on OpenHack.com (thanks to Tarsi for the link).