There are several types of firewall techniques:

In practice, many firewalls use two or more of these techniques in concert.

A firewall is considered a first line of defense in protecting private information. For greater security, data can be encrypted.

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eSecurity Planet 
A resource for daily information on e-security targeted to IT managers. The site provides users with information from a variety of sources, including experts at security product and services firms, and the consultants who follow the security industry.

Design the Firewall System
A practice from the CERT Security Improvement Modules.

Extinguishing Firewall Hyperbole
Are new firewall technologies improvements or complications? Evaluate the pros and cons.

Great Walls of Fire
Hackers, lurking cyber punks and viruses. Enough to give any network administrator the willies. But, an efficient firewall could be just the armor your network may need to combat most of these threats.(2001)

SecurityTracker.com
Information on the latest security vulnerabilities, free SecurityTracker Alerts, and customized vulnerability notification services.

Why You Need a Firewall
Describes security issues when connecting to the Internet, with information on how to protect your network. This page is hosted by Cisco Systems Inc.

Yahoo!'s firewall companies page
Yahoo!'s directory of firewall companies.

Yahoo!'s firewalls page
Yahoo!'s directory of firewalls.

Firewall Info
Protect your system with a firewall. You must protect your system with a personal firewall, especially if you are using DSL or cable modem.

Firewalls FAQ
Contains answers to basic questions about firewalls.