This is my own work. It is an article I originally wrote for my site on Internet / Network Security. Part of the article is included below as well as a link to the original at which has links to other sites and resources on the subject:

As you begin to learn the essentials of computer and network security you will encounter many new terms: encryption, port, Trojan and more. Firewall will be a term that will appear again and again. So, what is a firewall?

A firewall is basically the first line of defense for your network. The basic purpose of a firewall is to keep uninvited guests from browsing your network. A firewall can be a hardware device or a software application and generally is placed at the perimeter of the network to act as the gatekeeper for all incoming and outgoing traffic.

A firewall allows you to establish certain rules to determine what traffic should be allowed in or out of your private network. Depending on the type of firewall implemented you could restrict access to only certain IP addresses or domain names, or you can block certain types of traffic by blocking the TCP/IP ports they use.

There are basically four mechanisms used by firewalls to restrict traffic. One device or application may use more than one of these in conjunction with each other to provide more in-depth protection. The four mechanisms are packet-filtering, circuit-level gateway, proxy server and application gateway.

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Full Article: Introduction to Firewalls