Reduce unnecessary connectivity in a legacy operational technology (OT) network by first documenting what communicates and why, then grouping assets into operationally meaningful zones and allowing only necessary, monitored traffic between them. Put controlled intermediaries between IT and OT where data must cross, and introduce new boundaries through site-approved change control with a workable rollback plan. Segmentation can reduce exposure and improve control; it cannot guarantee that an incident will be prevented or that a generic firewall will work safely in every plant.
Why segmentation must be designed around the process
Network segmentation divides a network into separately controlled segments. Boundaries can reduce exposure and make permitted traffic easier to control. CISA describes it as a physical or virtual approach that divides a network into subnetworks, each providing additional security and control, in its January 2022 Layering Network Security Through Segmentation infographic.
In OT, those boundaries also have to preserve the communication patterns that keep industrial processes available and predictable. Legacy control environments may have limited internal segmentation or remote-access mechanisms, and their access arrangements may not behave like common IT setups. OT assets can also be difficult to replace on ordinary IT timelines because of operational constraints. Those realities make an inventory and a controlled rollout essential, rather than treating segmentation as a device-installation task.
1. Map assets and required communication before changing boundaries
Start by building an inventory and a communication map. Record each relevant asset’s role, operational criticality, dependencies, and remote-access paths, along with the traffic it needs for normal operation. Include communications that cross between business IT, control networks, and external or vendor access paths.
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The goal is to establish an evidence-based baseline before writing rules. An allowlist based on assumptions can block a dependency that was overlooked; a map grounded in observed and operationally confirmed flows gives the site a stronger basis for deciding what to permit. CISA’s recommended practices call for organizing assets into zones according to criticality, consequence, and operational necessity, then defining acceptable conduits between them.
Questions the map should answer
- Which systems perform control, monitoring, engineering, historian, or business functions?
- Which assets depend on one another for normal operation, and what communications support those dependencies?
- Which connections cross trust boundaries, including operator, engineer, vendor, or other remote access?
- Which flows are necessary, and which are merely present or not yet understood?
Do not turn an unexplained flow into a permanent exception by default. Identify its owner and operational purpose before deciding whether it belongs in the permitted design.
2. Draw zones around functions and consequences
Use the map to group assets into zones that reflect what they do, how critical they are, the consequences of compromise or outage, and their operational requirements. Then define the conduits—the permitted paths for communication—between those zones. A zone should represent a meaningful trust and operational boundary, not simply a convenient subnet or a copied diagram.
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A Purdue-style layered view can help organize business and control-system areas, as CISA’s defense-in-depth material illustrates, but it is a reference rather than a rule that every asset must be assigned mechanically to a level. The actual plant’s dependencies and communication paths should determine where boundaries belong.
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- Separate assets where their function, criticality, or consequence justifies distinct controls.
- Keep permitted conduits explicit: identify the communicating zones or endpoints, the needed protocol, and direction.
- Document why each permitted path is required and who is responsible for confirming that need.
- Prefer a small number of clearly understood, controlled paths over broad connectivity that is hard to review.
3. Put a controlled intermediary between IT and OT
Where information must pass between business IT and OT, use a DMZ or another controlled intermediary rather than unregulated direct communication. CISA’s recommended practices describe a DMZ as a way to eliminate unregulated communication between IT and OT. In practice, design the permitted exchange around specific hosts and connections: define which system initiates a transfer, what destination it reaches, and what information or service is required.
A DMZ is a boundary architecture, not a guarantee of safety by itself. Its effectiveness depends on the paths allowed through it and how those paths are controlled. Avoid treating a broad network route through an intermediate zone as sufficient just because it is not a direct IT-to-OT connection.
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4. Select boundary controls for the actual traffic
Firewalls, proxies, and gateways can enforce boundaries, and segmentation can be physical or virtual. The appropriate mechanism depends on the facility’s protocols, required communications, availability constraints, visibility needs, remote-access model, and the change burden of deployment. CISA’s guidance does not establish one universally suitable device or configuration.
| Mechanism | Role in a segmented design | What to assess at the site |
|---|---|---|
| Physical segmentation | Separates network segments through physical architecture; CISA identifies it as a segmentation approach. | Which paths remain between segments, what equipment or cabling changes are needed, and how the design affects availability. |
| Virtual segmentation | Separates network segments through logical architecture; CISA identifies it as a segmentation approach. | Whether the existing infrastructure supports the intended separation and whether policies can control the actual inter-zone flows. |
| Firewall, proxy, or gateway | Enforces or mediates traffic at a boundary; CISA recommends these as segmentation mechanisms. | Whether it supports the required OT traffic and protocol behavior, provides useful monitoring, and can be introduced and reversed under site change controls. |
| DMZ or other intermediary | Provides a controlled place for required communication between IT and OT instead of unregulated direct communication. | Which hosts and specific connections are needed, how they are controlled, and whether any route still permits broader access than intended. |
This comparison describes roles, not a product ranking. No product model or generic configuration can be selected from these principles alone. In particular, do not assume a standard IT firewall policy will safely handle a plant’s OT protocols or timing and availability requirements without site-specific analysis.
5. Write narrow rules and monitor the conduits
For each permitted conduit, define the minimum necessary source, destination, protocol, and direction. Record the operational reason for the rule rather than relying on broad permissions that are difficult to validate. Monitor inter-zone traffic so operators can detect unexpected communications and check whether the design matches real dependencies.
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Monitoring is useful both before and after enforcement: it can help expose flows that need investigation and show whether traffic continues to match the intended boundaries. It does not replace confirming requirements with the people responsible for the process, and observed traffic alone should not be treated as proof that every rarely used or exceptional operational path has been captured.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.6. Deploy changes through operational change control
Treat a new boundary or rule set as an operational change, not an ordinary network tidy-up. The site should decide how to validate the design and how to respond if expected communications fail. CISA’s material establishes the importance of understanding OT constraints and communication paths; the specific change procedure and test plan must be determined for the facility rather than assumed to be universal.
- Review the proposed boundary. Confirm the assets, required flows, remote-access paths, and operational owners represented in the design.
- Plan how to introduce it. Use the site’s change-management process to identify approval, timing, dependencies, and any required coordination with operations and engineering staff.
- Define success and rollback in advance. Specify what the site will check to confirm normal operation and how it will restore the previous configuration if the change causes an unacceptable effect.
- Validate the resulting behavior. Confirm the required communications and process functions after the change, and review monitoring for unexpected inter-zone traffic.
- Keep the design current. Document approved exceptions and revise the map and rules when assets, dependencies, or operational needs change.
These are prudent implementation steps for reducing the risk of disruption; they are not a single test procedure prescribed for every OT environment. The facility’s operating and change-management requirements govern the rollout.
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How to judge whether a design is suitable
Compare candidate approaches against the plant’s own needs rather than choosing by product category alone. Check boundary strength and the number of controlled paths; compatibility with required protocols and predictable communications; visibility into inter-zone flows; the equipment, configuration, and outage burden; rollback feasibility; and how vendor and operator access will be mediated, authenticated, authorized, and audited.
A design is not ready to enforce if important flows remain unexplained, necessary access paths are undocumented, or the site has no approved way to validate and recover from the change. Resolve those gaps before tightening boundaries. Segmentation is one layer of defense in depth, not a substitute for operational ownership, monitoring, or other security controls.
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