Use the tutorials on this site. I was clueless on the world of subnetting until I read the tutorials on Firewall.cx. You may have to read them a couple of times to really let it sink in. Also, it really helps is you have a network you can play with and test out some of the techniques.
Like everyone says, try the tutorials on this site because they are plain and simple. I think they are easier to understand then the books they have on the subject. If you can get some computers and a router or two you can practice the concepts. GL