Firewall
/ˈfaɪər.wɔːl/ · Noun · Security & Infosec · Origin: 1988
Definitions
A firewall is a network security system, implemented in hardware, software, or both, that monitors and filters incoming and outgoing traffic based on defined rules. It enforces a security boundary between trusted internal networks and untrusted external ones like the internet. Firewalls range from simple packet filters that inspect IP addresses and ports, to stateful firewalls that track active connections, to next-generation firewalls (NGFWs) that perform deep packet inspection and application-layer filtering. Cloud providers offer virtual firewalls through security groups (AWS), network security groups (Azure), and VPC firewall rules (GCP). Web application firewalls (WAFs) protect specifically against HTTP-level attacks like SQL injection and cross-site scripting.
In plain English: A digital bouncer that sits between your computer and the internet, deciding which data gets in and which gets blocked.
Metaphorically, a firewall is any barrier that prevents unwanted information or influence from crossing a boundary. In organizations, a 'firewall' between editorial and advertising means business interests can't influence content decisions.
Example: 'We need a firewall between the sales team and the engineering backlog — feature requests should go through product.'
Source: metaphorical
Etymology
- 1988
- DEC engineers publish first paper on network firewall filters
- 1993
- First commercial firewall products appear
Origin Story
Literal walls of fire that saved buildings before they saved networks
The original **firewall** is an architectural feature: a fire-resistant wall built between sections of a building (or between row houses) to prevent flames from spreading. The concept dates back centuries — after the Great Fire of London in 1666, building codes began requiring brick party walls between structures.
The metaphor made the jump to computing in the late 1980s. The idea was identical: place a barrier between a trusted internal network and the untrusted outside world to prevent "fire" (unauthorized access) from spreading in. The first commercial network firewall, DEC SEAL, was shipped in 1992. But the conceptual groundwork was laid by Bill Cheswick and Steve Bellovin at AT&T Bell Labs, whose 1994 book *Firewalls and Internet Security* became the bible of the field.
The automotive world also uses the term: a firewall is the metal barrier between the engine compartment and the passenger cabin, protecting occupants if the engine catches fire. All three meanings — architectural, automotive, digital — share the same core idea: keep the dangerous stuff on one side.
Coined by: Adapted from architectural terminology
Context: Late 1980s network security community
Fun fact: The Great Wall of China served a similar 'firewall' function for the entire nation. Some researchers have nicknamed China's internet censorship system the 'Great Firewall' as a deliberate double allusion.