When your computer is connected to the Internet, it receives traffic from a wide range of sources, most of it benign. Your instant messaging client alerts you that a friend has signed on; your mail client finds new mail waiting for you and downloads it; a weather site refreshes its rainfall map by telling your web browser to reload a page. All of this traffic is handled invisibly by your computer, which is listening to a large number of "ports." A port is a specific connection point through which applications on your computer connect to the Internet. And a hacker only needs one open port through which to mount an attack.
Your computer is just one machine among the millions connected to the Internet at any given moment. And a moment is all it takes for a hacker to get in

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