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What is a “Firewall”?

Answer
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In its most basic definition, a firewall is a barrier between your PC and the Internet. Regardless of whether you are using a dial-up or a broadband connection such as cable or DSL, the moment you connect to the Internet, your PC becomes vulnerable to attack. This is due to the fact that your PC becomes “registered” on the Internet with its own unique address called a TCP/IP address. Using this address, unscrupulous individuals on the Internet can get into your computer to get at your resources and/or data – unless of course you are running a firewall.

To best understand the firewall, a brief understanding of TCP/IP addresses and TCP/IP ports is required. This is most easily accomplished with an illustration. Let’s assume that your PC is an apartment building – a large building with many people, each living in their own apartment. If someone wanted to send a package to one of these people living within your apartment building, they would have to specify the postal address that gets the package to the proper building in the proper town. The postal address equates to the TCP/IP address. Now that the courier is at the correct building, he needs to deliver the package to the appropriate person within the building. For that, they read the apartment number off of the delivery label on the package. The apartment number equates to TCP/IP port numbers.

To summarize, your PC has a unique TCP/IP address that allows it to be found on the Internet. Within that TCP/IP address, there are many TCP/IP ports – in fact there are 65,535 TCP/IP ports.

A hacker or other individual looks for the easy target. They start by running a program called a “port scanner” or “port probe”. This program simply tries to communicate with a TCP/IP address to see if a computer responds. If so, the port scanner then begins trying each of the 65,535 TCP/IP ports at that TCP/IP address looking to see if one is “open”, the equivalent of an unlocked door. If found, the port scanner program alerts the hacker so that they may then try to come though this open door and look around or cause trouble.

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