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The Internet: The Missing Manual by David Pogue, J.D. Biersdorfer

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Sealing Your Computer's Firewall

Believe it or not, there are even more bad things that can happen when you're online. Total strangers, next door or in Eastern Europe, can connect to your Windows PC, invisibly take control of it, and turn it into, for example, a relay station that helps them pump out millions of pieces of spam (junk email) every day. You might notice that your PC has slowed down, and you might not. But you've just become part of the problem.

How is this possible? To understand the technical underpinnings, you need to know about ports.

Ports are like TV channels. Your PC has a bunch of them, each one dedicated to letting certain kinds of Internet information pass through: surfing the Web, sending email, downloading files, playing videos, and so on. Trouble is, Internet intruders roaming around online know how to use these ports to their advantage. They use software that can slip into your PC through one of these ports.

Ready to yank your modem cable out of the wall yet? Relax. You can stop the baddies just by using a firewall, a security barrier that prevents people or programs from sneaking into your machine via your Internet connection. A firewall can be a software program or a physical piece of hardware.

Good firewalls can monitor both incoming and outgoing traffic. So, in addition to keeping out intruders, your firewall can detect—and stop—spyware or a virus trying to transmit information from your computer.

Hardware Firewalls

A hardware firewall is a physical ...

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