Purdue Model
A reference architecture for industrial control systems that divides a plant into hierarchical levels — Level 0 field devices (sensors and actuators), Level 1 basic control (PLCs and DCS controllers), Level 2 supervisory control (HMIs and SCADA), Level 3 site operations (historians, MES), a demilitarised zone, and Levels 4-5 enterprise IT. Originally a manufacturing integration model, it is now used as the basis for network segmentation in OT security, and underpins the zone-and-conduit model in IEC 62443. The Purdue Model is examined in IEC 62443 and most OT security curricula.
Why It Matters
In practice the Purdue Model matters because it gives OT and IT teams a shared vocabulary for where a control system may and may not talk, and the industrial DMZ between Level 3 and Level 4 is where most defensible OT architectures actually live: enterprise systems pull data from a replicated historian in the DMZ rather than reaching down into the control network. Flat plant networks where a compromised business laptop can route directly to a Level 1 controller are how incidents escalate from IT nuisance to physical consequence. The model is a reference and not a blueprint, and two realities complicate it — modern IIoT sensors and cloud analytics deliberately bypass the hierarchy, and many brownfield sites have decades of undocumented cross-level connections. Real segmentation work usually starts with passive asset discovery to find out what the network actually is, because the diagram on the wall is rarely accurate. On exams such as IEC 62443 and OT security certifications, expect questions on which devices sit at which level, why the industrial DMZ exists, how conduits between zones should be controlled, and why availability and safety outrank confidentiality in this environment.
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Related Network Security terms
Firewall
A network security system that monitors and controls incoming and outgoing network traffic based on predetermined security rules, acting as a barrier between trusted internal networks and untrusted external networks. Firewalls can be hardware appliances, software applications, or cloud-based services. Types include packet-filtering, stateful inspection, application-layer (proxy), and next-generation firewalls (NGFW) that combine traditional filtering with intrusion prevention and deep packet inspection. Properly configured firewalls are the first line of defense in network security and are essential knowledge for Security+, CCNA Security, and CISSP certifications.
VPN (Virtual Private Network)
A technology that creates a secure, encrypted connection (tunnel) over a less secure network such as the internet, allowing remote users to access private network resources safely. VPNs use protocols like IPSec, SSL/TLS, WireGuard, or OpenVPN to encrypt data in transit. Common implementations include site-to-site VPNs connecting office networks and remote-access VPNs for individual users. Split tunneling allows users to route only corporate traffic through the VPN while personal traffic goes directly to the internet. VPN concepts are heavily tested in Security+, CISSP, and network security certifications.
IDS (Intrusion Detection System)
A device or software application that monitors a network or systems for malicious activity or policy violations and generates alerts when suspicious behavior is detected. IDS can be network-based (NIDS), monitoring traffic on network segments, or host-based (HIDS), monitoring activity on individual systems. Detection methods include signature-based (matching known attack patterns), anomaly-based (detecting deviations from normal behavior), and heuristic analysis. Unlike an IPS, an IDS only detects and alerts, it does not actively block threats. Popular IDS tools include Snort, Suricata, and OSSEC.
IPS (Intrusion Prevention System)
A network security tool that monitors network traffic flows to detect and actively prevent identified threats in real time. Unlike an IDS which only alerts, an IPS sits inline with traffic and can drop malicious packets, block connections, or reset sessions automatically. Modern IPS solutions use signature matching, anomaly detection, and behavioral analysis to identify attacks. They are often integrated into next-generation firewalls (NGFW) as a unified threat management feature. IPS is a key component of defense-in-depth strategies and is tested in Security+, CySA+, and CISSP certifications.
DMZ (Demilitarized Zone)
A physical or logical subnet that separates an internal network from untrusted external networks, providing an additional layer of security for public-facing services. Servers in the DMZ (such as web servers, email servers, and DNS servers) are accessible from the internet but isolated from the internal network by firewalls on both sides. If a DMZ server is compromised, the attacker still cannot directly reach internal resources. DMZ architecture is a classic example of defense-in-depth and network segmentation. It is a common topic in Security+, CISSP, and network architecture certifications.
SIEM (Security Information and Event Management)
A software solution that aggregates and analyzes security data from across the organization, including logs from firewalls, servers, endpoints, and applications, to detect threats and support incident response. SIEM platforms provide real-time alerting, correlation of events across multiple sources, dashboards, and compliance reporting. Leading SIEM tools include Splunk, Microsoft Sentinel, IBM QRadar, and Elastic Security. SIEM is the backbone of modern Security Operations Centers (SOCs) and is essential knowledge for CySA+, CISSP, and CISM certifications.