The Role of NAC in Securing OT (Operational Technology)

In an era where cyber threats are growing more sophisticated and industrial systems are becoming increasingly connected, protecting Operational Technology (OT) environments is no longer optional—it’s critical. From manufacturing floors to power plants, OT networks are full of specialized devices that were never designed with security in mind. That’s where Network Access Control (NAC) comes in.

While NAC has long been used in traditional IT environments to manage user access, its value in OT security is rapidly gaining recognition. When implemented properly—especially with passive techniques like DHCP gleaning and MAC address clustering—NAC can dramatically improve visibility, reduce risk, and enforce policy without disrupting critical operations.

The Unique Security Challenges of OT Environments

OT systems are designed to prioritize availability and uptime above all else. They often involve:

• Legacy devices (e.g., PLCs, SCADA systems)
• Proprietary protocols
• Minimal or no built-in security
• Static configurations with limited change tolerance

Unlike IT systems, many OT devices can’t handle active scanning or probing without risk of crashing or interfering with industrial processes. This makes traditional endpoint protection and vulnerability management tools ineffective—and even dangerous—in OT settings.

So how do you secure something you can’t touch?

Enter Network Access Control

NAC solutions provide a non-intrusive, policy-based framework for managing what devices are allowed to connect to the network, where they can go, and how they behave. In OT environments, NAC offers:

• Visibility: Discover and identify every device on the network
• Control: Apply policies to restrict access based on device type, risk, or role
• Segmentation: Isolate critical systems and prevent lateral movement

All without requiring agents, scans, or direct interaction with fragile OT equipment.

Why Passive Fingerprinting Matters

Because many OT devices can’t tolerate active scanning, NAC solutions  that use passive methods for identifying and classifying endpoints are critical

Two of the most powerful passive techniques in this context are:

DHCP Gleaning: The Key to Safe, Silent Discovery

DHCP gleaning involves capturing information from DHCP requests to learn about new or existing devices on the network. Every time a device connects and requests an IP address, it sends a packet with valuable data such as:

• MAC address
• Hostname
• Device vendor
• Operating system hints
• DHCP options (like device type or role)

This process is:

• Agentless
• Non-disruptive
• Ideal for real-time device discovery

In OT environments, DHCP gleaning provides instant awareness when new or unauthorized devices appear—without touching the device or interfering with its operation.

MAC Address Clustering: Profiling Devices Through Behavior

MAC address clustering is another powerful technique that enhances device fingerprinting. By grouping devices based on similarities in:

• MAC address prefixes (revealing vendor/type)
• Network behavior (e.g., traffic patterns, protocols used)
• Port location or VLAN

NAC can infer what type of device is connecting, even if it provides minimal DHCP information. For example:

• A set of devices with similar MAC prefixes, static IPs, and Modbus traffic patterns are likely to be PLCs.
• A device in a known location with similar traits to other HMIs can be classified—even without login credentials or rich metadata.

This clustering allows for context-aware access control, even in environments with legacy or silent devices.

Benefits of NAC in OT Environments

Let’s bring it all together. Here’s what NAC delivers to operational networks:

1. Real-Time Visibility

You can’t protect what you can’t see. NAC provides a live inventory of all connected devices, including unmanaged and third-party equipment. This is essential for both security and compliance efforts.

2. Reduced Attack Surface

By enforcing device-based access policies, NAC ensures that only known, trusted devices can connect—and that they’re only accessing what they should. Rogue devices are blocked or isolated automatically.

3. Safer Network Segmentation

NAC helps implement microsegmentation based on device type, risk level, or business function. For example, you can keep your building management system on a separate VLAN from your production line.

4. Incident Response and Forensics

With a full history of device activity, MAC address relationships, and policy logs, NAC enables faster incident responseand helps pinpoint the origin of any network anomalies.

5. Passive Monitoring that Respects OT Stability

Most importantly, NAC does all this without disrupting sensitive OT systems. It leverages passive fingerprinting, not active interrogation—keeping your processes running smoothly and securely.

Operational networks are no longer isolated—and that means they can no longer remain unsecured. As IT and OT converge, organizations need a way to enforce strong access control, gain visibility, and reduce risk—without endangering uptime or safety.

Network Access Control offers exactly that, especially when enhanced with smart, passive techniques like DHCP gleaning and MAC address clustering. These methods give security teams the insights and control they need to protect industrial infrastructure—without putting it at risk.

If you’re looking to modernize your OT cybersecurity posture, NAC isn’t just a nice-to-have—it’s a must-have.