Leaving a parked car unlocked and running on a busy street in the city spells trouble. Of course, no one in his or her right mind would ever do that. But are you doing it right now with your computer network? If you don't have a firewall in place, you sure are.

Every time you connect to the Internet via dialup modem, DSL, cable modem or T1 line, you link your computer with all the other computers that are also connected to the Internet. Without a firewall, your computer, plus any others on your network, become vulnerable to attack. That means computer hackers can pretty easily break into your network and use your computing systems to attack other companies or steal sensitive data like customer credit card numbers.

Rather than having your network be so accessible, a firewall acts as a choke point. It works by monitoring all inbound and outbound Internet traffic and blocking any traffic that has been deemed inappropriate based on what you have specified. A multitude of firewall options exists for small office networks; implementing any one larger than a personal firewall is generally a task best left to the technically proficient.

Just make sure you don't skimp when it comes time to install a firewall. Use firewall experts who have extensive experience setting one up or risk having a firewall that is, despite the good intentions that went into building it, as leaky as a sieve. Also, invest in the appropriate computer equipment, especially RAM. Since the firewall will be the single point through which all traffic travels, you will want to ensure that traffic can be processed quickly.

While you may be tempted to use the firewall to censor your staff's activities by blocking access to sites, resist the urge. There are many ways to circumvent this blocking that it would not be worth the time to set up these blocks. Setting a firm policy about acceptable usage can be a more effective way to influence employee behavior.

Firewall pricing ranges from free to tens of thousands of dollars. If you only have one computer that connects to the Internet, a personal firewall, which costs about $50, should be fine. Small office networks where multiple computers have access to the Internet should look for a more robust solution that can cost a few to several thousand dollars, including the cost of set-up.

When comparing costs, make sure to factor in the costs of support. One solution may be initially more costly, but may save you far more in the long run when factoring in the cost of maintenance visits to reconfigure or reboot your firewall. To get a handle on post-installation costs, find out about how reliable the firewall is, make sure it can handle your expected traffic, and learn when visits will be necessary.

Although no firewall is 100% reliable, having one can be enough to deter a bored hacker from going for a joyride with your computer network and instead turn to another, hassle-free network to take over.