If you really want to understand various security controls in Linux based systems, then I highly recommend checking out gentoo or Linux From Scratch so that you can install a system from the core components without any built-in security controls. This will allow you to hand configure the controls you wish to implement and will force you to watch the vulnerability lists to maintain a secure systems. The greater purpose is to force you to use that system as your primary system so that the necessary research and testing is forced upon you.
You should also checkout backtrack http://distrowatch.com/table.php?distribution=backtrack so that you can test and learn about various network and physical intrusion methods.
Can you share what aspect of Network Security you are most interested in such as packet analysis, port control, remote exploits, intrusion detection systems, etc...?