This book discusses many of the security tools you can use to defend your computer against digital threats. An antivirus program and anti-spyware programs are critical; however, the third key tool in your defenses should be a firewall, which can be used to keep out intruders and traveling viruses (called worms). A firewall is an electronic wall used to keep out an intruder or unwanted communication. It sits between your computer and the Internet (see Figure 3.1).
Figure 3.1. A firewall is an electronic barrier between your computer or your home network and the Internet.
In construction, a firewall is a physical wall that is designed to stop the spread of fire from one part of a building to another. Firewalls are also used in vehicles to separate the engine compartment from the passenger compartment . In the case of a network, a firewall stops unauthorized communication from the public Internet to a computer.
Think of a firewall as a big wall with lots of doors in it. These doors are called ports. When you use your web browser to access a website such as Google.com, you open a port (an electronic door) from inside the firewall and make contact with a server (a computer that sends information on request).
Let's say the server belongs to Google. Now that you have made contact with Google, it is allowed to communicate back to you. So data flows two ways through the port because you have initiated contact first and the firewall knows this.
There are potentially thousands of ports on a firewall. For example, email goes through port 25. Web browsers use port 80, and MSN Messenger uses ports 1863, 6891-6900, and 6901.
You don't have to know what ports are used. Programs on your computer figure that out for you. Some programs, such as file-sharing applications, however, need ports to be specially configured to work.
Crackers use port-scanning software to look for holes in a firewall. These scanners yell out to your firewall on various ports, "Yo dude, are you there?" Your computer would normally respond, "Yep, I am here. Ready to yak." The port scanner then knows that there's an opportunity to exploit a security hole and crawl through that door.
There are two basic types of firewalls at your disposal: