What is a firewall?

A 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.

Sources

• National Institute of Standards and Technology — NIST CSRC Glossary: https://csrc.nist.gov/glossary

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