What is a Firewall?

A firewall is a system that controls access between your own network and the outside world by using a set of pre-configured rules or filters.

The term 'firewall' can refer to a piece of hardware specifically designed to act as a firewall or it can be a software program that is installed on a piece of equipment (e.g. a laptop or a server) as a firewall.

As well as controlling incoming access to your network, firewalls can be used to control external sites that your network users can access.

A firewall cannot give 100% security on it's own - it should always be used in conjunction with other protection technologies such as an antivirus and anti spam in order to keep your network as secure as possible.

There are two basic rules by which a firewall allows or denies access:-

• Everything is denied unless specifically permitted
• Everything is permitted unless specifically denied

The first, where everything is denied unless permitted, is recommended most often as, by default, it provides a much higher level of security but it can be somewhat restrictive and even prevent necessary traffic until the correct rules and protocols are set up and configured.

The second, where everything is permitted, is less secure but much more flexible as it only prevents access to areas specified by the administrator.

Both these rules have three levels:

• Level 1 is basic filtering
• Level 2 is intermediate filtering
• Level 3 is advanced filtering

One way to look at it is as if your private network is an island surrounded by a series of security fences which get higher and stronger:

• The 1st fence is made up of several software programs - antivirus, anti spam etc
• The 2nd fence is level 1 of your firewall
• The 3rd fence is level 2 of your firewall
• The 4th fence is level 3 of your firewall

Your firewall is like a passport control guard who checks your passport to see if it is valid - if it is valid then you can get to the other side of the security fences, if it is not valid, you are not allowed to go through the fences.