Application Level Gateway Devices

A router does not have the software sophistication to act as an application level gateway.

A gateway firewall requires a PC or Unix workstation computer with a server operating system (OS). The gateway itself is not part of the OS but is separate software running on top of the operating system.

Traditionally Unix has been the OS used for building firewalls. Usually the Unix has been hardened, that is, cleaned up and made more secure. There are also special secure OSes built specially for running firewalls. More recently, people have built firewalls on top of Windows NT.