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What is a Firewall?

W

hen 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.

A firewall is a piece of software that monitors all incoming network traffic and allows in only the connections that are known and trusted.

You could manually grant or restrict access to each of the 65,535 ports available under the Internet Protocol. Every time you add a new program that requires Internet access, you would need to determine which port(s) it uses, and reconfigure your computer accordingly. There are better ways to spend your time.

Firewall software takes on this burden for you, allowing access to the ports you need open, and closing off those you don't. It also makes your computer "invisible" on the Internet; if hackers can't find you, they will have a hard time attacking you.

More advanced firewall software also monitors outgoing traffic. This is crucial since malicious code spreads by accessing the Internet and pushing copies of itself to other computers (often those of your friends and family!). Outbound protection can keep even brand-new Trojan horses and spy-ware from doing their damaging work. The ultimate protection is program-level control, so that only those applications that you trust are allowed to access the Internet.

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