there are some courses given at universities or collges under continuing education for that type of stuff.
if u wanna learn on ur own though, you could aim for a certification, where you buy a book, read it, do exercises, and then go to an exam online at a test center. something like network +, or ccna , would be a good introduction to basic/intermidiate networking. then you could go for something more security oriented.....i cant think of the acronym atm.
otoh, if u just wanna learn some basic hacking, theres a good book, "hacking windows 2000" and the sort, where the authors try to go over some of the valnurabities hackers usually exploit.
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