Firewalls and Access Control

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.

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What is a Firewall?

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.

Firewall Placement

                    ┌─────────────┐
   Internet ◀──────▶│  Firewall   │◀──────▶ Internal Network
                    └─────────────┘
                          │
                    ┌─────────────┐
                    │     DMZ     │
                    │ (Web, Mail) │
                    └─────────────┘

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Types of Firewalls