Firewalls are categorized as different types based on how they deal with network traffic and the layer of the OSI model at which they operate (if you need a refresher on the OSI model, take a look back at Chapter 5, "Network Protocols: Real and Imagined"). The higher the layer of the OSI model at which the firewall operates, the more sophisticated the firewall. Here are some specifics:
You will find that some firewall products are actually a hybrid of the types of firewalls discussed here. For example, a firewall might combine circuit-level capabilities with application-gateway services.
The deployment of firewalls and the development of new firewall products are both proceeding at seemingly breakneck speed. Major news stories on the cracking of big corporate and government networks have only lead to a rise in the purchasing of firewalls and the manufacturing of more secure firewall products.
You will find that most of the network operating systems available also provide some sort of firewall protection. This includes NetWare, Linux, and Windows Server 2003.