A firewall’s basic task is to regulate the flow of traffic between computer networks of different trust levels. Typical examples are the Internet which is a zone with no trust and an internal network which is a zone of higher trust. A zone with an intermediate trust level, situated between the Internet and a trusted internal network, is often referred to as a "perimeter network" or Demilitarized zone (DMZ).

A firewall’s function within a network is similar to firewalls with fire door in building construction. In former case, it is used to prevent network intrusion to the private network. In latter case, it is intended to contain and delay structural fire from spreading to adjacent structures.

Without proper configuration, a firewall can often become worthless. Standard security practices dictate a "default-deny" firewall ruleset, in which the only network connections which are allowed are the ones that have been explicitly allowed. Unfortunately, such a configuration requires detailed understanding of the network applications and endpoints required for the organization’s day-to-day operation. Many businesses lack such understanding, and therefore implement a "default-allow" ruleset, in which all traffic is allowed unless it has been specifically blocked. This configuration makes inadvertent network connections and system compromise much more likely. (See Wikipedia, Firewall, http://en.wikipedia.org/wiki/Firewall)

Comodo Firewall Free

System: Windows 2000/XP/2003/Vista

Licence: Freeware

Comodo Personal Firewall offers truly excellent protection, on a par with the better for-pay personal firewalls.

• Constantly monitors and defends your PC from internet attacks
• Free maintenance upgrades
• Gain complete control over which programs are allowed internet access
• Firewall stay protected against new threats via automatic online updates
• Real time traffic monitoring enables you to react instantly to threats and identify bandwidth bottlenecks.
• Easy to use interface and quick setup
• No license fee - complete protection at no cost for networks and homeusers

Comodo Firewall Pro is one of the most impenetrable personal firewalls on the market. There are many reasons why its technology is so powerful:

• Unlike other firewalls, Comodo Firewall Pro does not need a single configuration change to enable its full strength.
• It classifies more than 10,000 applications according to their risk level such as SAFE, SPYWARE, and ADWARE.
• It uniquely passed 16 serious 3rd party leak test utilities with an outstanding success rate not seen in any other firewalls. It also passed Comodo’s Patent Injection Leak Test which emulates a hacker technique typically used in outbound Trojan attacks.
• Windows security center integration
• Full Application Behavior analysis
• More powerful and intuitive security rules interface
• Completely redesigned interface including "Smart" pop-up alerts
• Advanced network, application and application component monitoring

In addition, the Comodo Firewall Pro helps with connection details like IP addresses and ports. It also provides free proactive technical support.

HomePage: http://www.personalfirewall.comodo.com

ZoneAlarm Free Firewall

System: Windows 98se/Me/2000/XP/Vista

Licence: Freeware

ZoneAlarm is designed to protect  DSL- or cable-connected PC from hackers. This program includes four interlocking security services: a firewall, an application control, an Internet lock, and Zones. The firewall controls the door to your computer and allows only traffic you understand and initiate. The Internet lock blocks Internet traffic while your computer is unattended or while you’re not using the Internet, and it can be activated automatically with your computer’s screensaver or after a set period of inactivity. Firewall Zones monitor all activity on your computer and alert you when a new application attempts to access the Internet.

HomePage: http://www.zonelabs.com

Sunbelt Kerio Personal Firewall

System: Windows 98/ME/NT/2000/XP

Licence: Freeware

Sunbelt Personal Firewall can run in a free mode vs. a full (paid) mode. Install it now, and for the first 30 days it will run in ‘full’ mode. After that, it shuts down selected features, but will continue to run in ‘free’ mode".

• Stop all traffic – stops all traffic on the computer. This function can be helpful especially when undesirable or strange network activity is detected. Traffic can be restored after the appropriate security actions are taken.
• Logging – Each firewall module creates an independent log that is stored in a text file. Logs can be viewed in a configuration dialog.
• Connections overview and statistics – The overview provides information about established connections and ports opened by individual applications. Information on the current speed and size of transmitted data in both directions is also provided for active connections. The overview is automatically refreshed in predefined time intervals. Statistics show users the number of objects blocked by the Web content filter and the number of detected intrusions during specific time periods.
• Automatic update – Regular checks are made for newer versions of the firewall. Whenever a new version is detected, users have the option of downloading and installing it.

HomePage: http://www.sunbelt-software.com

PC Tools Firewall Plus

System: Windows 2000/XP/2003/Vista

Licence: Freeware

PC Tools Firewall Plus is advanced technology designed especially for people, not just experts. Powerful prevention against attacks and known exploits is activated by default while experienced users can optionally create their own advanced packet filtering rules, including IPv6 support, to customize the network defenses. All you need to do is install it for immediate and automatic ongoing protection.

• Firewall protects your PC as you are working, surfing and playing.
• Intelligent automatic protection without all the questions.
• Easy to use. Designed for both, novice and expert users.
• Advanced rules to protect your PC against common attacks.

HomePage: http://www.pctools.com/firewall

Ashampoo FireWall FREE

System: Windows 2000/XP

Licence: Freeware

• Easy Mode / Expert Mode
• Easy: One-click firewall protection for everyone
• Expert: Activate expert functions if you need them
• Configuration Assistant guides you through the setup process
• Amazingly small and compact, firewall uses minimum memory and computer resources
• Learning Mode identifies applications that try to phone home
• Block All panic button to stop all traffic with a single click
• Detailed logging of all connections
• Four additional network security tools included in the tiny package

HomePage: http://www.ashampoo-root03.com

GhostWall Firewall

System: Windows 2000/XP/2003/XP64

Licence: Freeware

• Ideal for low latency applications
• Doesn’t impact online games (Counter-Strike,World of Warcraft,etc)
• Low resource and CPU use
• Shows blocked attacks and country of origin
• 64-bit (x64 / XP64) compatible

HomePage: http://www.ghostsecurity.com/ghostwall/

R-Firewall

System: Windows 98/2000/XP/2003

Licence: Freeware

R-Firewall protects a computer connected to a local network and/or to the Internet against any intrusions, attacks, trojans, spyware, and other external and internal threats. R-Firewall also filters out inappropriate Internet pages content and blocks dangerous active components you may get while browsing over the Internet or receiving e-mail massages.

• Firewall monitors network activity at the application and system component levels;
• Detects and blocks direct network intrusions;
• Detects and blocks network connections that malicious programs try to establish from within and outside the protected computer;
• Detects and blocks numerous types of attacks;
• Detects and blocks various dangerous active content on the Internet, such as JavaScript objects, ActiveX components, etc;
• Checks e-mail attachments for dangerous content, such as executable programs, JavaScript objects, ActiveX components, etc;
• Converts the protected computer into a "stealth" machine running invisibly to outside intruders;
• Provides detailed log information

HomePage: http://www.r-firewall.com

Filseclab Personal Firewall

System: Windows 95/98/ME/NT/2000/XP/2003

Licence: Freeware

• Filseclab Personal Firewall can block the most attacks from worm viruses and trojans.
• Filseclab Personal Firewall can block some main Adware and Spyware, the definition supports live update and you may also define it yourself.
• Double filter system provides double-layer protection.
• Digital signature verification can automatically trust the well-known program.
• Built-in 7 big modes can apply for all kinds of requirements.
• Advanced realtime monitor lets the network activities be clear.
• Interactive rules creator is very easy to create the rules.
• Password protection can protect the rules and configurations.
• Filseclab Personal Firewall is very easy to backup and restore the rules.
• Can control the websites easily.
• Firewall supports two logfile types, ascii and binary.
• Also supports live update for rule-definition, traffic graph, privacy protection, Windows Security Center, balloon message alert and more unique features.

HomePage: http://www.filseclab.com

Outpost Firewall Free

System: 95/98/ME/NT/2000

Licence: Freeware

Outpost protects your system from worm and virus attacks that arrive by e-mail. You can filter e-mails based upon their content, and Outpost will prevent dangerous attachments from executing.

HomePage: http://www.agnitum.com

iSafer Winsock Firewall

System: Windows NT/2000/XP

Licence: Freeware

This is a Firewall which runs in socket layer - Winsock firewall is a filter which checks all the network packages against the specific rule set. Winsock firewall can be used to check and block Spywares, Adwares, etc.. running in your PC. With developers, this is a good example of basic firewalls. It is too simple for a developer to understand the architecture of the firewall.

HomePage: http://isafer.sourceforge.net

Soft Perfect Personal Firewall

System: 95/98/ME/NT/2000/XP/2003

Licence: Freeware

SoftPerfect Personal Firewall is a free network firewall designed to protect your PC against attacks from the Internet or via a local area network. SoftPerfect Personal Firewall offers customizable security using user-defined rules for packet filtering. It supports multiple network adapter configurations. Firewall is easy to use, comes with a set of predefined rules and it is completely free. Additional features include password protection, and logging.

HomePage: http://www.softperfect.com/products/firewall/