The Sasser worm has once again showed why a firewall is so important when you are on the net. A firewall is software, or hardware, that restricts what is allowed to go between your computer and the net. It lets you decide what, if anything, can come in or go out of your computer.

When your computer connects to the Internet, or any network, it gets an address. Along with the address it opens ports, which are doorways to allow that data to come in and out. A firewall is a program that locks the doors and allows only what you want to enter and leave your computer.

If your computer is connected directly to the Internet, then you need a software firewall. Windows XP and Apple OS X come with a built in firewall. The latest version of XP comes with that firewall enabled. If you have an older version of Windows XP then Microsoft have the instructions for setting it up here. Apple also have instruction on setting up the OS X firewall.

Third party firewalls are better than the built in XP firewall because they give you more control. If you are running Windows 98, ME or an older Mac then a third party firewall is necessary. You can buy these from the local computer shop or download a free firewall such as Sygate or Zone Alarm. Unfortunately we don't know of any free firewalls for Macs, if you do, tell us.

Routers and the built in XP and OSX firewalls will simply stop potential intruders from testing your ports. Most software firewall will nag you about which programs are allowed to connect to the net. Once you've told it the programs you want to connect to the net, such as email, web browsers and antivirus updates, it will stop any unauthorised program like spyware or viruses.

If you are using a router to share Internet access by definition it has basic firewall functions, in that your computers aren't directly connected to the net. More advanced routers will have specific firewall features you can configure to allow certain programs to access the net. Some of the newer home routers have quite sophisticated firewall and blocking features, read the manual carefully before setting them up.

It doesn't matter. Worms like Sasser and MSBlaster can infect you anyway. Dial up users are less attractive to hackers and similar people, but they are still vulnerable. Anybody who connects to the net needs a firewall.

Because a firewall's job is to block network traffic, it can cause a lot of problems. If you have a home or office network, a firewall will block it unless you set it up correctly. Also, many programs will stop working unless you allow them to access the net. Some Internet providers will stop working as well.

A common problem is software being blocked, if you are running a web set, using instant messaging or logging into corporate networks, then you will have to tell your firewall to let certain traffic through. If you find a firewall is causing problems, first check the manual for the firewall and the affected software. It might just be a matter of changing a setting.

Some firewalls decide they will not behave with any software and can cause freezes and crashes, if this happens, uninstall and replace the firewall.

Despite the complexity, firewalls are an important part of your protection on the net. Without a firewall, there is the possibility that others can connect to your computer. It's important to keep computers as secure as possible, so make sure you have a firewall.

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