fire wall

In architecture, a fire wall (or firewall) is a wall constructed of non-combustible materials—masonry, brick, stone—designed to prevent fire from spreading between sections of a building or between adjacent structures. The concept is ancient: Roman insulae (apartment blocks) used party walls to slow conflagration, and medieval European cities, repeatedly devastated by fire, mandated fire-resistant construction after disasters. The Great Fire of London (1666) accelerated the adoption of firewall requirements in building codes across Europe.

American building codes formalized the term 'firewall' in the late 19th and early 20th centuries, specifying exactly how thick, how high, and of what materials these separating walls must be. A firewall in construction must extend through the roof line and sometimes above it to prevent fire from jumping across the gap. The principle is containment through separation: you cannot stop fire from starting, but you can stop it from spreading.

When computer networks began connecting to each other in the 1980s, security engineers needed a metaphor for the barrier between a protected internal network and the untrusted external one. 'Firewall' was borrowed from architecture between 1987 and 1988—the term appeared in a 1988 paper by Digital Equipment Corporation engineers describing their packet filtering system. The metaphor was exact: a firewall in a building stops destructive elements from spreading; a network firewall stops malicious traffic from crossing a boundary.

Today, firewalls are the most fundamental element of network security. Every router, every enterprise network, every cloud infrastructure runs some form of firewall. The word has traveled so far from its architectural origin that most people who configure firewall rules have never thought about brick and mortar. But the logic is unchanged across two millennia: build a non-combustible barrier between what you want to protect and what might destroy it.