Firewalls provide protection against outside attackers by shielding your computer or network from malicious or unnecessary Internet traffic. Firewalls can be configured to block data from certain locations while allowing the relevant and necessary data through (see Understanding Denial-of-Service Attacks and Understanding Hidden Threats: Rootkits and Botnets for more information). They are especially important for users who rely on "always on" connections such as cable or DSL modems.
Firewalls are offered in two forms: hardware (external) and software (internal). While both have their advantages and disadvantages, the decision to use a firewall is far more important than deciding which type you use.
Most commercially available firewall products, both hardware- and software-based, come configured in a manner that is acceptably secure for most users. Since each firewall is different, you'll need to read and understand the documentation that comes with it to determine whether or not the default settings on your firewall are sufficient for your needs. Additional assistance may be available from your firewall vendor or your ISP (either from tech support or a website). Also, alerts about current viruses or worms (such as US-CERT's Cyber Security Alerts) sometimes include information about restrictions you can implement through your firewall.
Unfortunately, while properly configured firewalls may be effective at blocking some attacks, don't be lulled into a false sense of security. Although they do offer a certain amount of protection, firewalls do not guarantee that your computer will not be attacked. In particular, a firewall offers little to no protection against viruses that work by having you run the infected program on your computer, as many email-borne viruses do. However, using a firewall in conjunction with other protective measures (such as anti-virus software and "safe" computing practices) will strengthen your resistance to attacks (seeUnderstanding Anti-Virus Software and other security tips for more information).
Both the National Cyber Security Alliance and US-CERT have identified this topic as one of the top tips for home users.
Authors: Mindi McDowell, Allen Householder