No discussion of firewalls would be complete without a discussion of the different types of firewalls. This is particularly true in this context, because it allows you to better understand exactly where in the spectrum the free firewall offerings lie. In the networking sense, a firewall is basically any component (software or hardware) that restricts the flow of network traffic. This is a sufficiently broad definition to allow for all of the various ways people have chosen to implement firewalls. Some firewalls are notoriously limited in capability and others are extremely easy to use.

Within the realm of firewalls there are many different ways to restrict network traffic. Most of these methods vary in the level of intelligence that is applied to the decision-making process. For example, to permit or deny traffic based on which network device is the sender or recipient, you would use a packet-filtering firewall. In reality, even the simplest packet filtering firewalls can typically make decisions based on the source Internet Protocol (IP) address, the destination IP address, and the source and/or destination port number. While this type of firewall may sound overly simplistic, consider if you have a server running a Web site for use on the Internet. In all likelihood, the only traffic that you need to allow to the server uses a destination port of Transmission Control Protocol (TCP) 80 or 443; thus, you could configure your firewall to permit only that traffic. These ports are used for HTTP and...