What is a Firewall?

One of the things computers do very well is communicate with each other. This is accomplished through a variety of mechanisms, one of which is the network, where computers are physically connected to each other through cables or equipped with transmitters giving them the ability to wirelessly exchange data.

On the Internet, for example, each computer connected to the network is issued an address: a unique number that identifies that computer so other machines know where to route the data that computer has requested. One of the easiest ways to understand how a firewall works is to think of each of those computers as a hallway containing several doors.

The technical term for these "doors" is a "port." Each port, or door, also has a number assigned to it, with each different number corresponding to a different kind of data. For example, port 80 is used for web pages, while port 23 is used for e-mail. When you requested this article, for example, your computer issued a request through "door 80" on your machine, and the page was delivered to that same door by the server where the page resides.

The problem with this system is that any other computer on the Internet that knows your computer's address can send data through those ports. Since it is important to make sure nothing malicious like a virus ends up on your machine, it is a very good idea to equip all of the "doors" on your computer with locks.

That, simply put, is all a firewall really is. It allows the owner of a computer system to lock the doors, or ports, in and out of their system so no unauthorized data can pass. Many firewalls monitor data moving in both directions, which can also help prevent problems with spyware, which often transmits data from individual computers, or "phones home," sending that data back to a central server.

Without a firewall in place, it is possible for a malicious program to simply ask a randomly accessed computer "what programs are running and on what ports?" or "which doors are unlocked?" Once they have this information, they can gain access to that remote machine through one of those unlocked doors or open ports. This can all happen automatically without the remote computer's owner having the slightest idea it is occurring.

With a firewall in place, all the remote computer will find are locked doors. Most firewalls have a default setting which simply denies any and all remote access regardless of what port the request is meant for. This makes sense for the average home user, since inbound traffic from remote machines is almost always meant for servers instead of desktops. There are very few circumstances under which a home user would need to open any ports other than for web pages or e-mail, and even then, only the outbound ports.

No computer, no matter the brand, operating system or manufacturer, should ever be connected to the Internet without a functioning firewall. It is likely and quite possibly certain that an unprotected computer would be infected with all manner of viruses, trojans or spambots within minutes of being connected.