firewall
termsecuritynetworking
A device or function that decides which traffic may pass between networks, enforcing a policy at a boundary.
The name is borrowed from a wall built to stop fire spreading between parts of a building, and the metaphor carries the assumption that has aged worst: that there is an inside and an outside. It was true when an organisation had one gateway, and it stopped being true with cloud, remote work and machine-to-machine traffic that never touches the perimeter. The function did not become useless; it multiplied and moved, into hosts, hypervisors, service meshes and cloud policy, which is what segmentation and zero trust are actually describing. The warning from the people who wrote the first book still applies: an estate where the firewall is the only secure component is one attackers go around rather than through.
Also known as: ngfw