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A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11Separate enterprise IT from operational technology (OT) through a controlled demilitarized zone (DMZ), then divide OT into zones based on operational function and consequence. Permit only necessary, defined communications between zones, and monitor those boundaries. This can restrict an intruder’s ability to move laterally or reach essential control systems, but segmentation is one layer of defense—not a guarantee against intrusion.
What OT network segmentation does
OT networks connect systems that monitor or control physical processes, such as industrial control systems (ICS), supervisory control and data acquisition (SCADA) systems, programmable logic controllers (PLCs), human-machine interfaces (HMIs) and field devices. A flat network can give a compromised device more paths to other systems than it needs. Segmentation creates boundaries between groups of systems; rules at those boundaries limit which communications can cross.
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The goal is not to block every connection. It is to make each permitted path deliberate, limited to an operational need and observable. CISA says segmentation can help contain an intrusion and prevent or limit lateral movement in its StopRansomware Guide. It reduces exposure; it does not make a network immune to ransomware or other attacks.
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Design zones and conduits around operations
Start with the processes the network supports, their criticality and the consequences of disrupting them. Group assets with related functions and security needs into zones. A conduit is the defined communication path between zones. For each conduit, decide which systems may communicate, for what operational purpose, and how that traffic will be filtered and monitored.
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Separate enterprise IT and OT rather than treating the corporate network as trusted for control-system security. CISA, the FBI and the NSA recommend using a DMZ between IT and OT to prevent unregulated communication, and advise: “Prohibit ICS protocols from traversing the IT network.” That does not mean all OT information exchange must stop; it means connections should be designed and controlled rather than allowing direct, unrestricted paths. See the agencies’ January 11, 2022 advisory.
| Design decision | Less restrictive pattern | More controlled pattern | Why it matters |
|---|---|---|---|
| IT-to-OT connection | Direct, unregulated communication | Separate IT and OT with a DMZ between them | A DMZ gives the organization a boundary at which to control and observe permitted exchanges. CISA, FBI and NSA advisory |
| OT organization | One broad trusted OT segment | Multiple zones grouped by function, criticality and operational need | Compromise of one system need not automatically provide the same access to every OT asset. CISA, FBI and NSA advisory |
| Traffic between zones | Broad or implicit access | Explicitly permitted, filtered communications | Boundaries can block unnecessary paths while allowing validated operational dependencies. CISA segmentation infographic |
| Boundary visibility | Unmonitored connections | Logged and monitored boundary traffic | Monitoring helps reveal unexpected communication and supports incident response. CISA segmentation infographic |
Segmentation may use physical separation, logical separation, or a combination. The appropriate design depends on the site’s assets, dependencies and operational requirements; the CISA infographic’s layered network examples illustrate concepts, not a ready-to-deploy plant design. Purdue-style levels can help describe system functions, but they are not a substitute for a current asset map and site-specific risk assessment. CISA’s older ICS defense-in-depth practice describes zones and treats the enterprise zone as untrusted for ICS security because of its wider connectivity and exposure.
Plan and implement segmentation in stages
- Inventory assets and dependencies. Record OT and IT assets, owners, functions, criticality, communications dependencies, and remote or third-party access. Include relevant cloud connections. An inventory that omits a device or dependency can lead to a boundary rule that disrupts operations or leaves an unrecognized path open.
- Map current connectivity. Document the existing topology, major networks, addressing, interdependencies and connections among enterprise IT, DMZs, OT operations and control devices. Identify uncontrolled paths and devices that connect or bridge multiple segments. CISA’s StopRansomware Guide recommends current network diagrams and secure documentation, including offline backups or hard copies for incident response.
- Agree on safe operating requirements. Work with process owners and control-system engineers to identify the functions that must continue, the systems they depend on and the communications required for safe operation. Do this before changing network paths.
- Define zones and permitted conduits. Group assets by operational function, criticality, consequence and necessity. For every proposed connection across a zone boundary, specify the communicating systems and the permitted traffic needed for the validated purpose. Avoid rules broader than that purpose requires.
- Establish the IT/OT boundary. Separate enterprise IT from OT and use an intermediary DMZ to control permitted exchanges. Apply filtering and monitoring at boundaries. Do not allow direct, unregulated enterprise-to-control-system communication.
- Validate, then change incrementally. Check the proposed design against documented dependencies. Make changes in stages, observe traffic and confirm that control and safety functions continue to work before tightening or removing paths. Involve qualified OT/ICS engineers and the people responsible for the process when validating production changes.
- Keep the design record current. Update diagrams and access documentation when approved changes are made. Protect the records and retain an offline copy or hard copy for incident response, following the documentation guidance in CISA’s StopRansomware Guide.
There is no universal firewall rule set or safe change procedure for every OT site. Rules must follow the validated operational dependencies of the specific environment. A network firewall can filter traffic at a boundary, but choosing an appliance or writing rules alone does not establish a safe segmentation design; compatibility and operational effects need engineering validation. CISA describes boundary controls in its segmentation infographic.
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Prepare for isolation and recovery
Segmentation should be paired with an incident plan that accounts for the possibility that enterprise IT may need to be isolated from OT. Identify which processes must continue, who can authorize isolation, and what tested manual controls or workarounds are available. Exercise those procedures so operators understand how to maintain safe operations if normal IT services or network connections are unavailable.
Maintain isolated backups and test them regularly as part of recovery planning. CISA’s OT ransomware fact sheet discusses continuity, workarounds and backup resilience. Segmentation can limit pathways and impact, but it should sit alongside access controls, monitoring and response and recovery procedures. CISA’s segmentation infographic explicitly notes that segmentation is not the only tool to secure a network.
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