Type of Firewalls

There are three basic types of firewalls depending on:

* Whether the communication is being done between a single node and the network, or between two or more networks.
* Whether the communication is intercepted at the network layer, or at the application layer.
* Whether the communication state is being tracked at the firewall or not.

These network-layer and application-layer types of firewall may overlap, even though the personal firewall does not serve a network; indeed, single systems have implemented both together. There's also the notion of application firewalls which are sometimes used during wide area network (WAN) networking on the world-wide web and govern the system software. An extended description would place them lower than application layer firewalls, indeed at the Operating System layer, and could alternately be called operating system firewalls.