Network segmentation limits which devices, services, and applications can communicate with industrial control systems (ICS). To use it effectively, map assets and operational dependencies, divide the environment into zones that reflect function and consequence, route necessary exchanges through controlled conduits, and monitor the boundaries. The design must preserve safe, reliable operations: a firewall or DMZ alone does not create a complete segmentation strategy.
What segmentation does in an industrial environment
Network segmentation divides a network into physical or virtual subnetworks and limits access between them. CISA describes it as a way to constrain access to devices, data, and applications. In an industrial environment, the practical goal is to prevent a problem in one part of the network from automatically becoming a problem everywhere else.
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Without meaningful boundaries, an intruder or compromised device may be able to communicate broadly with systems that were never intended to interact. Segmentation narrows those paths: a device in one zone should reach another zone only through an approved conduit, using a documented protocol or service for a defined purpose. Boundaries also create places to observe and investigate traffic.
Segmentation can limit opportunities for lateral movement and help contain the effects of an intrusion, but it does not prevent every intrusion or guarantee a particular reduction in damage. CISA’s January 2022 Layering Network Security Through Segmentation infographic says that properly implemented DMZs and firewalls can shield high-value assets from unauthorized access. The qualification matters: the design, configuration, monitoring, and operation of the controls determine whether the intended boundary exists in practice.
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Think of a boundary as both a restriction and an observation point. A rule that blocks an unnecessary connection can reduce exposure, while logs and monitoring at the same crossing can help teams notice attempted or unexpected communication. Neither function is automatic: teams need to know what should cross, configure controls accordingly, and review what is happening in operation.
Design zones around process reality, not a diagram
Start with operational function, interdependence, criticality, and the consequences of compromise or interruption. Assets that perform related roles may belong together; systems whose compromise could have substantially different consequences may warrant separate zones. A generic reference model can help organize a discussion, but it is not a ready-made plant design.
A layered view may distinguish enterprise IT, an intermediary DMZ, manufacturing or supervisory systems, control or cell environments, and field devices. Those labels are useful only if they describe real communication relationships and support the plant’s operational requirements. CISA ICS-CERT’s 2016 defense-in-depth guidance describes layered ICS zones and conduits; CISA, FBI, and NSA guidance emphasizes zones based on role, criticality, consequence, and operational necessity.
For each proposed zone, ask what trust boundary it represents and what would be gained by separating it. A useful answer connects the zone to a function, the systems that depend on it, or the consequences of unauthorized access or interruption. Avoid drawing boundaries solely to make a diagram look layered: the result may be complicated to maintain without meaningfully restricting a risky path.
Make the IT/OT boundary explicit
Establish a clear boundary between enterprise IT and operational technology (OT). CISA, FBI, and NSA’s January 11, 2022 advisory recommends a DMZ that eliminates unregulated communication between IT and OT, and advises prohibiting ICS protocols from traversing the IT network. The agencies’ wording is: “Define a demilitarized zone that eliminates unregulated communication between the IT and OT networks.”
A DMZ is an architectural intermediary, not a protective layer simply because it exists. If it has broad routes, permissive rules, or poorly understood connections, it can still allow excessive access. Put only the necessary exchange services there, constrain which endpoints can reach them and how, and monitor the crossings. The aim is controlled communication—not an assumption that every IT/OT interaction can or should be eliminated.
During planning, distinguish the business need for information exchange from the network path currently used to satisfy it. The existing path may be broader than the need. Identify which systems must exchange information, what service supports that exchange, which direction it needs to travel, and whether it can be mediated through a controlled intermediary. Do not preserve an unregulated direct path just because it is familiar; equally, do not remove a dependency before the responsible operational and engineering teams understand its function.
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Separate OT functions where the consequences justify it
Within OT, consider whether manufacturing, supervisory, control, cell, and field-device environments need distinct boundaries. A zone should represent a useful trust or operational boundary, not just a convenient subnet. For example, separate a system from neighboring environments when its criticality, function, dependencies, or compromise consequences make a shared zone inappropriate. Conversely, dividing tightly interdependent systems without understanding their required exchanges can interrupt operations.
A Purdue-style diagram is a reference for thinking about layers, not a substitute for site-specific flow analysis. CISA’s March 2022 energy-sector guidance recommends layered architecture and functional segmentation, while also discussing one-way devices where possible. Neither source establishes a universal number of zones or a universal configuration for every facility.
Asset criticality should inform—not replace—dependency analysis. A highly sensitive asset may merit tighter restrictions, but its zone still has to account for legitimate operational communications. CISA’s segmentation considerations also connect inventory to data sensitivity and internet access. Those details help teams decide which assets need especially careful treatment and where external exposure or cross-zone access warrants attention.
Build an initial segmentation plan in six steps
1. Establish the current state
Inventory the systems and connections that could affect or communicate with the control environment. Include controllers, HMIs, engineering workstations, servers, network devices, remote-access services, third-party connections, and relevant IT systems. Record the links between them, the users and services involved, the protocols and direction of communication, and the business or process purpose.
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For each asset, capture details that help determine its placement and protection needs: operational role, criticality, data sensitivity, dependencies, and whether it has internet access. CISA’s segmentation guide connects asset inventory to decisions about criticality, data sensitivity, and internet exposure. A list of device names alone is not enough; the objective is to understand what must communicate for the process to operate.
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Include paths that may be easy to overlook when drawing a network: remote-access services, third-party connections, and devices attached to more than one segment. Record who uses each access path and what work it supports. An inventory that omits these paths can make a proposed boundary look more complete than it is. Where the purpose of a connection is unclear, note the uncertainty and identify who can resolve it rather than silently treating the flow as either essential or unnecessary.
2. Define zones by trust, function, and consequence
Draw the enterprise IT/OT boundary first, then group OT assets according to role, interdependence, criticality, and the consequences of compromise or interruption. Identify high-value or especially sensitive systems that may need a more restricted zone. Mark assets that are connected to more than one segment, since they can create paths across boundaries that the network diagram may otherwise suggest are isolated.
For each proposed zone, write down its purpose, included assets, owner, and the operational dependencies that must be supported. If you cannot explain why two systems are in the same zone or which interactions are needed across zones, the design is not ready to enforce.
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3. Define and approve conduits
For every proposed cross-zone connection, document the initiating device, destination, protocol or service, direction, purpose, owner, and operational dependency. Distinguish a required connection from one that merely exists today. Remove unnecessary access, and route necessary IT/OT exchanges through a DMZ or another controlled intermediary.
Rules should identify approved endpoints and specific connections rather than broadly permitting a network to reach another network. If the purpose, owner, or operational dependency of a flow is unknown, resolve that uncertainty with the responsible engineering and operations teams before treating the flow as safe to remove or safe to retain.
A conduit register makes the design reviewable: each entry should explain not only what is allowed, but why it is needed and who is accountable for confirming that need. Record direction explicitly rather than assuming that a service’s name describes how traffic moves. Where an exchange has several endpoints or purposes, document them clearly enough that a reviewer can distinguish the approved use from an overly broad rule. This specificity gives the team a practical basis for checking that deployed filters match the intended architecture.
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4. Enforce the boundaries
Use appropriately configured firewalls and filtering controls on network devices to restrict communication between zones. Permit only the documented paths that have been approved for the intended devices and services. An industrial Ethernet firewall is one implementation category for enforcing approved traffic between zones; the appropriate product and placement depend on site protocols, traffic patterns, reliability requirements, and architecture. Buying or installing a firewall does not by itself establish a sound segmentation design.
CISA’s March 2022 guidance also recommends intrusion detection and monitoring at key chokepoints. Where the use case permits, a one-way communication device may be considered for a path that should carry information in only one direction. CISA says to use such devices “whenever possible”; that is not a universal substitute for documenting and evaluating required flows.
Boundary enforcement has to reflect the approved flow analysis. Check that filtering is applied where traffic actually crosses between zones, rather than assuming that logical separation alone blocks communication. Consider what happens if a rule is broader than intended or a control is misconfigured: the number and placement of boundaries should make those weaknesses easier to detect and correct, not hide them. The architecture should also leave teams able to understand and maintain the rules as equipment and processes change.
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Capture and review logs at zone boundaries, monitor traffic between segments and at remote-access services, and establish a baseline of expected ICS communications. Use the baseline to identify traffic outside normal operations, including unexpected endpoints, services, directions, or timing. An alert is a prompt to investigate against process context, not proof by itself that a system is compromised.
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Review firewall and filtering rules against the approved flow records. Check whether actual rules allow only the intended communications and whether logs provide enough detail to investigate a crossing. CISA’s guidance supports monitoring and baselining; it does not prescribe a universal alert threshold or a single monitoring product.
Monitoring is most useful when teams can connect an observed flow to an asset, an approved purpose, and an owner. If traffic does not match the baseline, first establish what systems communicated and whether there is a known operational explanation. An unexplained flow may point to a rule that was not documented, a changed dependency, or activity that merits further investigation. Keep monitoring focused on the boundary and remote-access paths where an unexpected crossing could reveal that the intended separation is not working as planned.
6. Validate changes with operations and maintain the design
Before changing production paths, review the proposed flows and failure behavior with control-system engineers and operations personnel. Confirm which functions depend on each connection, what happens if it is blocked or delayed, and how the plant will detect or respond to an unintended interruption. Stage changes in a way that allows the team to observe results and recover if an approved operational function fails.
CISA guidance establishes the need to account for operational necessity, but it does not provide a universal commissioning procedure. The facility must set its own safe validation and change-control process. Keep the network diagram, conduit definitions, rule ownership, and asset inventory current as equipment, suppliers, processes, and access arrangements change.
Validation should check both sides of the design intent: that required operations still work and that unapproved paths are no longer available. The appropriate sequence, timing, and recovery arrangements are site-specific; there is no universal sequence that can be safely applied to every control environment. Involve the people responsible for operations and control systems before a production change, and make sure the team knows how it will recognize an unintended effect.
After a change, reconcile the deployed configuration with the approved diagram and flow records. Assign ownership for review so rules do not become permanent simply because nobody knows why they were added. Revisit the design when equipment, suppliers, processes, or access arrangements change. This keeps segmentation tied to current operations rather than an outdated snapshot.
Choose controls by fit, visibility, and maintainability
Physical separation, logical segmentation, firewall-enforced boundaries, DMZs, and one-way communication devices address different design needs. CISA’s guidance does not rank physical controls above logical ones, or vice versa, for all facilities. Decide on a mix by evaluating the actual architecture and operational requirements.
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|---|---|---|
| Physical separation | Creates separation through distinct physical infrastructure. | Which communications must cross between the separated environments? How will those exchanges be controlled, observed, and maintained? |
| Logical segmentation, such as VLANs | Divides network environments logically while using shared physical infrastructure. | Where is traffic between segments actually restricted? What happens if configuration or access controls are misapplied? |
| Firewall-enforced boundaries and filtering | Restricts communications between zones to approved devices, services, and paths; boundary logs can support investigation. | Are the rules narrow, documented, and monitored? Can staff safely review and update them as processes change? |
| DMZ or controlled intermediary | Provides a defined place to mediate necessary exchanges between IT and OT rather than allowing unregulated direct communication. | Which services belong there, which endpoints may reach them, and which specific connections are permitted? |
| One-way communication device | Can support a use case in which information needs to pass in one direction. | Does the operational use case genuinely permit one-way communication? What required interactions would be affected? |
For any approach, assess containment (how many boundaries an attacker would have to cross and the consequences of a misconfiguration), operational fit, visibility at crossing points, alignment with asset criticality and dependencies, and maintainability. A design that is difficult to understand or update safely can lose effectiveness as the facility changes.
These are comparison axes, not a universal ranking. A physical boundary still needs its necessary exchanges to be controlled and observed; logical segmentation still depends on correct configuration and enforcement. A DMZ is useful when it mediates a real exchange, but its presence alone does not make that exchange safe. A one-way device is appropriate only where the use case permits one-way communication. Select the combination that matches the site’s dependencies and that staff can operate and review reliably.
There is no single cost figure or control package that applies to every facility in the guidance. The practical effort depends on the existing architecture, the assets and flows that must be understood, the controls needed at actual crossing points, and the work required to validate and maintain the design. Treat these as site-specific planning needs rather than assuming that purchasing a firewall is the whole project.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Failure modes that can undermine segmentation
- A permissive DMZ: An intermediary with broad or poorly controlled rules may still allow excessive access. Restrict endpoints and connections, and monitor crossings.
- Undocumented IT/OT paths: Broad routes or direct exchanges can undermine the intended boundary. Inventory connections and prohibit unregulated communication.
- Misconfiguration or user error: Incorrect rules or changes can open paths that the design intended to close. Review changes against documented flows and keep ownership clear.
- Devices connected to multiple segments: A dual-connected device can bridge environments and bypass the intended isolation. Identify these devices in the inventory and assess their role and configuration.
- Removable media and other non-network paths: Network boundaries do not control every way data or malicious code can enter an environment. Include relevant non-network paths in the facility’s broader security planning.
- Stale diagrams and rules: Equipment, suppliers, or processes can change while network documentation remains outdated. Reconcile diagrams, assets, and rules as part of ongoing maintenance.
Segmentation is one layer of defense, not a replacement for monitoring, access controls, patching, backups, incident response, or other protections. Its value depends on both the boundaries and the way the facility operates and maintains them.
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Do not judge the design only by how many zones appear on a diagram. Ask whether the important crossings are controlled, whether the rules are tied to documented operational needs, whether monitoring can show what crosses, and whether staff can safely maintain the controls. A complex design with poorly understood exceptions may offer less practical assurance than a clear design whose boundaries and dependencies are reviewed regularly.
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What a useful first-pass plan should contain
Before implementation, the operator and engineering team should be able to review a concise set of artifacts that turns the architecture into something testable and maintainable:
- An inventory of relevant ICS, OT, IT, network, remote-access, and third-party assets, including their roles and dependencies.
- A zone diagram showing the IT/OT boundary, any DMZ or controlled intermediary, and the proposed internal OT boundaries.
- A conduit register listing each required cross-zone flow, its endpoints, protocol or service, direction, purpose, owner, and operational dependency.
- A boundary-control plan identifying where traffic will be filtered, logged, monitored, or passed one way where appropriate.
- A validation and maintenance plan agreed with operations and control-system personnel, including how changes, diagrams, and rule ownership will be kept current.
These artifacts help expose gaps before a network change: unknown flows, unexplained access, dependencies that cross proposed zones, or paths that are permitted without a clear purpose. They also provide a basis for reviewing whether the deployed controls still match the intended design.
Use the artifacts together rather than as isolated paperwork. The inventory explains what is present; the zone diagram shows the intended trust boundaries; the conduit register records why communication crosses them; and the boundary-control and validation plans describe how the intent will be enforced, observed, and maintained. When one changes, check whether the others need to change as well.
A useful review asks whether every asset has a known role and owner, whether each cross-zone path has a documented purpose, and whether the proposed controls can support the required operation. It should also identify uncertainties that need resolution before a production change. That makes the first plan a working design for discussion and safe implementation—not merely a picture of the current network.
Frequently Asked Questions
Does a DMZ by itself secure the IT/OT boundary?
No. A DMZ is an intermediary for necessary exchanges, not a protective layer simply because it exists. Restrict which endpoints and specific connections can reach its services, monitor crossings, and avoid broad or undocumented paths. CISA, FBI, and NSA recommend a DMZ that eliminates unregulated communication between IT and OT.
Should ICS protocols cross the enterprise IT network?
CISA, FBI, and NSA’s January 11, 2022 advisory advises prohibiting ICS protocols from traversing the IT network. Plan necessary IT/OT exchanges through a DMZ or other controlled intermediary and document the approved endpoints, services, direction, and purpose.
Is physical separation always better than VLANs or firewall-enforced segmentation?
CISA’s guidance does not rank physical above logical controls for every facility. Compare approaches by containment, operational fit, visibility, asset criticality and dependencies, and maintainability. The site’s architecture and required communications determine the appropriate mix.
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No. CISA recommends one-way devices where possible, but they are suitable only when the operational use case permits information to move in one direction. They are not a substitute for mapping flows and confirming dependencies.
How many zones should an industrial control network have?
There is no universal number. Use zones that reflect real operational roles, interdependencies, criticality, and consequences of compromise or interruption. Purdue-style layers can help organize the discussion but are not a ready-made plant design.
Does segmentation replace other security controls?
No. Segmentation is one layer of defense and does not replace monitoring, access controls, patching, backups, incident response, or other protections. Misconfiguration, user error, removable media, or a device connected to multiple segments can also undermine isolation.
What should a facility review before changing production network paths?
Review the proposed flows and failure behavior with control-system engineers and operations personnel. Confirm dependencies, what happens if a connection is blocked or delayed, and how the facility will detect or respond to an unintended interruption. Use the facility’s own safe validation and change-control process; there is no universal commissioning procedure.
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Build segmentation from an accurate asset and flow inventory, define zones around operational function and consequence, and allow only documented, necessary communication across monitored boundaries. Use a DMZ to mediate required IT/OT exchanges, validate changes with operations and control-system personnel, and keep rules and diagrams current. Segmentation can limit lateral movement, but it is not a guarantee and works only as part of a maintained, layered security program.
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