What is Purdue Model?
Purdue Model is a reference architecture that divides industrial control system networks into a hierarchy of numbered levels, from physical field devices at Level 0 up to enterprise IT systems at Level 5. It is used to define security zones and control the flow of data between operational technology and information technology networks.
What it is
The Purdue Model, formally called the Purdue Enterprise Reference Architecture (PERA), was developed at Purdue University in the 1990s. It organizes industrial networks into six levels:
- Level 0: Physical process. Sensors, actuators, and field devices.
- Level 1: Basic control. PLCs, RTUs, and DCS controllers.
- Level 2: Supervisory control. SCADA systems and HMIs.
- Level 3: Site operations. Historians, batch management, and site-level control.
- Level 3.5 (DMZ): A buffer zone between OT and IT, often called the industrial DMZ.
- Level 4: Site business logistics. ERP systems and plant scheduling.
- Level 5: Enterprise network. Corporate IT systems.
The model defines which levels should communicate with each other and in what direction. Traffic between non-adjacent levels is restricted. The DMZ at Level 3.5 is the primary control point between OT and IT.
Why it matters
The Purdue Model gives security teams a shared vocabulary for describing where a device lives and what it should talk to. Without a reference architecture, OT networks often grow without clear boundaries, making it hard to contain a breach or audit traffic flows.
Regulatory frameworks including IEC 62443 and NERC CIP reference the Purdue Model's zone-and-conduit concept. Auditors and compliance teams use it to verify that segmentation controls are in place. Security architects use it to plan firewall rules, data diode placement, and access controls between zones.