What is a Firewall ?

A firewall is a piece of software that controls your computers ports and connections to the internet or any other sort of network.
There are 65535 ports available for a computer to make connections,some ports are specificly used for one service.
For example port 21 is always used for public FTP servers.
When your computer is connected to the internet without a firewall all ports are visible to the outside,
it's like opening your frontdoor and putting a sign next to it saying "I'm on holiday".
A computer connected to the internet without a firewall is an open invitation to others to connect to your computer 
and that means they can do ANYTHING on your computer.
So a firewall is essential if your computer is connected to the internet.


What does a firewall do ?

A firewall closes the open ports to the outside world (the internet) so hackers can't see and get in to your computer.


Do I need a firewall ?

If your computer is connected to the internet, yes.


So what firewall should I use ?

Depends,if you have one computer connected to the internet a FREE !!! software firewall like ZoneAlarm is perfect.
ZoneAlarm is easy to configure and offers good protection.
There are a few others like Sygate Personal Firewall,Kerio Personal Firewall and
Agnitum Outpost Personal Firewall but I don't have any experience with those.

If you have a home or office network a hardware firewall/router is the best solution.
You can either buy it in the shop for lots of money or if you are a bit handy with computers,build it yourself.
If you want to build one yourself you need a 386/486 or better,2 networkcards,1 floppydrive and only for configuration a monitor and keyboard.
For software you'll need a Linux based router like FreeSco or ClarkConnect,the simple versions of those don't even need a harddisk.


So I installed one,how do I know if it works ?

Make sure your firewall is OFF,
Click on one of these links...Shields Up from GRC.com,Sygate Online Services or
DSL Reports to start an online scan of your system.
You will probaly see some open ports show up in the scan,now start up your firewall and do the scan again.
Now all ports should be closed or stealth.


So now I'm safe from all attacks and virii ?

No.
A firewall mostly protects from attacks from the outside.
Virii and trojans are mostly activated from the inside by clicking on an email attachment for example.
If you have a virus or trojan installed most firewalls will not help much.
So install a virusscanner and scan every piece of software that comes into your computer,specialy email attachments.
Another way to get virii on your system is with floppies or cd-rom's so scan EVERYTHING before and after installing software.