A firewall is a device (or set of devices) that blocks network traffic based on certain rules and is used to prevent unauthorized access to the network. Many operating systems are now equipped with software firewalls. Most routers today can already perform some firewall functions.

Firewall technology emerged in the second half of the 1980s.

The name firewall came from the special wall (often metal) that in the past prevented fire from spreading to other parts of buildings (fire = fire, wall = wall).