Definition: firewall

The primary method for keeping a computer secure from intruders. A firewall allows or blocks traffic into and out of a private network or the user's computer. Firewalls are widely used to give users secure access to the Internet as well as to separate a company's public Web server from its internal network. Firewalls are also used to keep internal network segments secure; for example, the accounting network might be vulnerable to snooping from within the enterprise.

In the home, a personal firewall typically comes with or is installed in the user's computer (see Windows Firewall). Personal firewalls may also detect outbound traffic to guard against spyware, which could be sending your surfing habits to a website. They alert you when software makes an outbound request for the first time (see spyware).

In the organization, a firewall can be a stand-alone machine (see firewall appliance) or software in a router or server. It can be as simple as a single router that filters out unwanted packets, or it may comprise a combination of routers and servers each performing some type of firewall processing. For more about the various firewall techniques, see firewall methods. See WAF.

An Excellent Resource

O'Reilly's "Building Internet Firewalls, 2nd Edition" by Zwicky, Cooper and Chapman is one of the best books written on Internet and Web security. It covers a huge range of firewall and related topics and should be a "must have" for anyone interested in the subject. (O'Reilly & Associates, Inc., 2000)