# What is a Firewall?

> A firewall is a device, service, or software control that permits, rejects, or otherwise handles network traffic according to defined policy.

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## Content

Firewalls can sit between networks, protect an individual host, filter cloud traffic, or control communication between application workloads.

Firewall decisions may use addresses, ports, protocols, connection state, application information, identity, or other context. The security outcome depends on where the control is placed, what traffic it can inspect, how rules are governed, and whether required communications are understood.

In operation, the hard part is usually the policy rather than the appliance: rules accumulate until nobody remembers which flow each one serves. Mature management reviews rules regularly, removes shadowed and expired entries, ties every permit to a named business justification and owner, and treats emergency changes as temporary until re-reviewed.

### Key points

- **Primary purpose:** Enforce which network communications are allowed across a boundary.
- **Good practice:** Start from required flows, use least privilege, document ownership and purpose, log important decisions, and remove obsolete rules.
- **Common forms:** Packet filters, stateful firewalls, application proxies, host firewalls, cloud controls, and distributed workload firewalls.
- **Important limitation:** A permitted connection can still carry an attack, and a firewall cannot protect traffic that bypasses it or activity that never crosses its boundary.

### Related terms

[Next-generation firewall (NGFW)](<https://yellowcube.eu/glossary/next-generation-firewall/>) · [Network segmentation](<https://yellowcube.eu/glossary/network-segmentation/>) · [Web application firewall (WAF)](<https://yellowcube.eu/glossary/web-application-firewall/>) · [Zero trust architecture (ZTA)](<https://yellowcube.eu/glossary/zero-trust-architecture/>) · [Secure access service edge (SASE)](<https://yellowcube.eu/glossary/secure-access-service-edge/>) · [Proxy firewall](<https://yellowcube.eu/glossary/proxy-firewall/>) · [Unified threat management (UTM)](<https://yellowcube.eu/glossary/unified-threat-management/>)

### Sources

[NIST SP 800-41r1: Guidelines on Firewalls and Firewall Policy](https://csrc.nist.gov/pubs/sp/800/41/r1/final) · [NIST glossary: Firewall](https://csrc.nist.gov/glossary/term/firewall)

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