Understanding Firewalls 2. A firewall is a structure intended to keep a fire from spreading.
Building have firewalls made of brick walls completely dividing sections of the building.
In a car a firewall is the metal wall separating the engine and passenger compartments. Or, to keep the members of your LAN pure and chaste by denying them access the all the evil Internet temptations. The first computer firewall was a non-routing Unix host with connections to two different networks. You then used the resources of the system to access the Internet. For example, you could use X-windows to run Netscape's browser on the firewall system and have the display on your work station.