The best way to learn is NOT through dusty books, arcane RFCs, or strange burritos from Taco Bell. The best way to learn is to look at actual code, and see what it does. As any 3-5 year old can tell you, there is NOTHING to compare with just trying things out.
First, take a look at tcpdump/libpcap, the ancient and definitely definitive piece of code for monitoring IP packets. That'll give you a heads-up on how different packets are structured and what the different bits of structure mean.
Secondly, play around with the source for nmap. That shows you how portscans work, how fingerprints of OS' can be taken, and other good stuff.
Third, grab a copy of routed, a simple RIP router daemon. Why, you might ask? Security gurus might have an idea of what I'm up to. So, if you want to be an IP guru, you might want to make a few educated guesses. After all, the information is all contained above.
Fourth, grab hold of a few "less common" network tools. bing is kinda neat. There's some nice stuff in RAToolSet, too. I'm not just suggesting these for my health, they do actually have a value.
Lastly, go to SecurityFocus, or one of these other specialist on-line sites, and dig up everything you can find on honeypots.
Ok, now an explanation as to what is going on, here. Once you understand how to intercept packets, and/or inject them, you're in a strong position to protect yourself from any wanna-be intruders.
By learning how to massage the fingerprints of an OS, you can also learn how to send back misleading information to those who are interested in scanning your box. By this point, you can tell any would-be cracker whatever you wish.
The less-used tools can help identify the intruder, even if the intruder's computer is un-pingable. There still has to be router traffic, to tell the Internet where that machine is. There still has to be an AS number for that particular segment of the network. If you're -REALLY- lucky, someone may have set a nearby router to report its geographic location.
Finally, the bit the A-TEAM loved to do, on those cheesy TV shows. You set a trap. A honey-pot. In short, you set up something SO inviting, that when the skript-kiddies detect it, they'll plunge in. And then you've got them.
It doesn't have to be painful for them, it just needs to be (a) a way to keep them out of your hair, whilst you're doing real stuff, and/or (b) a learning-experience, on why not to be malicious to others.