# What is a firewall?

A firewall is a device or program that decides which network connections are allowed and blocks the rest, based on rules you set. It usually sits between your office network and the internet, and sometimes between internal segments, for example between office PCs and production machines. A rule might say: computers in the office may reach the internet on web ports, but nobody outside may reach the file server. A firewall is essential, but it only judges connections against its rules; it does not notice an intruder who uses a connection it allows. Network detection watches what happens inside those allowed connections.

FAQ: [What is…? Network and security terms](https://netsenx.com/en/faq/glossary) · https://netsenx.com/en/faq/what-is-a-firewall

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