Inspired by the release of Nmap 3.5, the latest version of the award-winning network security tool, I've been exploring network security issues for a couple of weeks now. Nmap's major skill is port scanning -- learning which ports on a machine are "open" and what applications are using them. Sound network security planning dictates that you take a look at your network machines to see what the bad guys can see from outside.

Think of a port as being an extension of a machine's IP address. The IP address itself equates to a street address for an apartment complex. The port number equates to a specific apartment. When packets are sent across the Internet, they include the port number they are destined for as well as the machine address.

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