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Smashing Coding[^]:

In the construction industry, a “firewall” is a specially-built wall designed to stop a fire from spreading between sections of a building... and in the late 1980s it made its way into computing. On one side of the wall is the seething electronic chaos of the Internet. On the other side is your powerful but vulnerable Web server. These computer firewalls are actually more like fire doors because they have to let some stuff through. They monitor all the electronic traffic coming in and out of a network. They follow a strict set of rules to determine what is allowed and what is blocked. This article explains in more detail how they work, the different types of firewalls available, what they are good at and not so good at, and how to configure them to protect a typical Web server.