# Firewall

A security system that monitors and filters incoming and outgoing network traffic based on defined rules.

A **firewall[↗](/firewall)** is a security system, either hardware or software, that monitors and filters network traffic based on a set of defined rules, deciding what is allowed to enter or leave a network or device. It acts as a checkpoint between a trusted internal network (like your home or office) and untrusted external networks (like the internet).

Modern firewalls range from the simple one built into your operating system to sophisticated "next-generation" firewalls used by businesses that can inspect traffic content, block known malicious sources, and enforce detailed rules per application. A firewall is an important layer of defense, but it isn't a complete solution on its own, it works best alongside other measures like antivirus[↗](/antivirus) software, patching, and strong authentication[↗](/authentication).