What is a Firewall?

A firewall is software that controls the flow of traffic between computers.

At its most complex, a firewall can control exactly the traffic, sites and users that can send data into and out of a network. At its simplest, a firewall merely prevents traffic entering a system from the outside. They are therefore an important part of a system's protection. However, firewalls can only protect you from direct attacks against your computer through the internet; they won't prevent a virus arriving in email, or downloading malcode through a web browser.

Firewalls can be included in the operating system (such as Windows XP/2003), a standalone product (such as Microsoft ISA Server) or included in a hardware device such as an ADSL router. In some cases you might use more than one firewall, say, a hardware-based firewall on the perimeter and software-based firewalls on each server.

• Tip! If you are planning to switch from a dial-up internet connection to ADSL, be sure to obtain an ADSL router with an in-built firewall rather than a standalone ADSL modem.