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Firewalls are the most fundamental network security control. They act as gatekeepers, deciding which traffic is allowed to enter or leave a network. Combined with access control lists (ACLs), firewalls enforce the principle of least privilege at the network level.
A firewall is a security device — hardware, software, or both — that monitors and filters network traffic based on a defined set of rules. It sits at the boundary between trusted and untrusted networks.
┌─────────────┐
Internet ◀──────▶│ Firewall │◀──────▶ Internal Network
└─────────────┘
│
┌─────────────┐
│ DMZ │
│ (Web, Mail) │
└─────────────┘
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