Firewall
Definition
A firewall is a hardware or software control that evaluates network traffic against policy rules and allows, denies, or logs flows accordingly.
Key Points
- It enforces traffic policy at a boundary or zone edge.
- It can filter by address, port, protocol, or application context.
- It is a primary control for segmentation and access restriction.
- It works best when paired with clear zone design.
- It is foundational in both enterprise and industrial perimeter security.
Concept
A firewall is not just a block list. It is a policy enforcement point that turns architecture into behavior. By constraining which flows can cross a boundary, it reduces the attack surface and helps operators prove that only approved communication paths are active. In OT systems, that role is often as important as packet filtering itself.
Explainer
Firewalls are essential where flat networks would otherwise allow unrestricted lateral movement. They do not eliminate risk, but they force traffic to declare its intent at the boundary. That makes them a core control in layered security models such as the Purdue architecture and in any environment where segmentation is used to protect critical systems.