What IS a firewall - What a firewall is and does.

The internet has undergone extreme growth in the last 10 to 20 years. What started out as a small military network known as the Arpanet quickly grew from a very simple text-based bulletin boards into the incredible global network it is today.

Web sites, online games, internet shops and even streaming movies have all found a home on the internet and there is almost no limit to the possibilities.

But with growth comes complexity. Spyware, scumware, worms, trojans are all signs of this wonderful new age. The sheer vocabulary needed to wrap our minds around the topic of the internet is staggering. Terms like spyware scanners, TCP/IP, 802.11, Ethernet, Flash, PHP and virtually hundreds of thousands more are used all over the web regularly not even bothering to be explained by the people that use them.

To make matters worse terms like browser cookies, web cache, trojans, worms and viruses are completely and differently defined for the internet (or the world of computers in general). Cookies are no longer for consumption, worms are no longer found in the ground and a firewall is not a fireproof wall to prevent the spread of fires.

But there's a few terms and expressions we should become familiar with and by now you should have already encountered things like a search-engine, a web-browser and a virus scanner.

It's hard to set priorities in the world of computers, each term seems as important as the next. Thankfully we are willing to learn as much as we can in order to make a start, we start with something that you will come to need and rely on if you're not already using one:

The Firewall

As mentioned before a firewall in the world of computers isn't something you would build around your computer in order to keep it from being destroyed by fire. Instead a firewall is considered to be one of the most important means to defend your computer from threats that lurk on the internet.

We have all heard of viruses but nothing seems to be even remotely as scary as the prospect of being attacked by a hacker (someone looking to invade and possibly take control of your computer).

To protect ourselves from these kinds of attacks companies and home users alike use a tool known as a firewall.

Rather than forcing ourselves to understand technical details of different types of firewalls and their different uses we will take a look at how the run-of-the-mill firewall manages to protect us from outside attacks.

At this point it is important to note that the explanation below is a toned-down example of what actually happens. If we were to go into technical details we would also have to discuss proxies, ports and head straight into the fairly complex world of TCP/IP. Way to much technical information!

Since we're not looking to make our lives any more complex than they already are we will stick to the basic concept of IP filtering, a technique employed by most (if not all) firewalls today.

All activities on the internet can be considered 'information' ... this "information" is sent all over the internet to make it possible for people to download files, view web pages and many other things.

Whenever you activate your internet browser and go to a web site you send information to that web site requesting information from that site in order to be able to display the web page.

To know what information has to go where and who is sending the information every computer on the internet has been given a unique number to identify themselves. This number is obtained through an internet provider (the people who charge you insane amounts of money so you can be online) and is automatically given to you when you sign in.

This unique number is known as an IP address (Internet Protocol address) and can be considered to be similar to the combination of street name, number, postal code, city and country information for the house you currently live in.

Not all information transmitted over the internet is desirable, so it would be nice if we could pick and choose what information we keep and what information conveniently gets lost (much like bills get 'lost' occasionally).

A firewall does just that. By looking at the IP address of incoming information the firewall can decide if the information that is trying to enter our computer is something we asked for or is something that we should 'lose'. Any information we don't want or we didn't ask for is simply blocked before it can enter the firewall. And unlike ignored letters (such as bills) information that is ignored on the internet will generally not come back to haunt you.

Other than filtering IPs the firewall also closes all your ports.

What's a port? Ports are like little doors into and out of your computer and are used for the various internet services like web browsing or email.

A more technical explanation:
Even though your computer has only one cable connecting you to the internet this one connection is made up out of over 65000+ little 'doors' (i.e. ports) that allow our different programs to connect to the internet. Every program that connects to the internet uses a specific door called a port to send out and receive information. Your internet browser for example uses port 80, a fileserver often uses port 21 and a different program will yet again use a different port.

In order to prevent people attacking you from the internet via one of those little doorways the firewall closes all these for you and in some cases will even camouflage them making them undetectable to the outside world.

It will then only allow your programs to make use of ports providing a good defense against unknown and unseen threats.