glossary
What is a firewall?
Clear guidance for learning that moves work forward
Use the 3-question Fit CheckA firewall is a device or program that controls the flow of network traffic between networks or hosts with different security needs, allowing or blocking traffic according to a defined security policy.
What it means
NIST defines a firewall as a gateway or control point that limits access between networks based on a security policy. In practice it acts as a filtered doorway: it lets approved traffic through and blocks unauthorized connections. Firewalls are a foundational concept in networking, systems administration, and security work.
Core source records
This table lists the page’s core content records and when they were checked. Claim-specific citations appear beside the relevant text and may not be repeated here.
| ID | Supports | Source | Checked |
|---|---|---|---|
| CIT-01 | Definition source for What is a firewall? | National Institute of Standards and Technology — NIST CSRC Glossary | 2026-06-15 |
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