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Use both. Network segmentation defines which PLCs, remote-I/O adapters, HMIs, engineering stations, safety systems and servers belong in the same trust zone. An industrial firewall, router, Layer-3 switch or security gateway then controls the conduit between those zones. A perimeter firewall alone leaves a flat control network vulnerable to lateral movement; VLANs without inter-VLAN policy are only labels.
Design the zones and conduits through an IEC 62443 risk assessment, then select enforcement devices that meet the plant’s availability, protocol, latency, redundancy and remote-access requirements. In most multi-cell plants, the practical pattern is a risk-based cell/area architecture, an industrial DMZ between enterprise and control networks, and explicit filtering at every important boundary.
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What an industrial firewall does
Policy enforcement at a boundary
An industrial firewall is a security control placed on a conduit between networks or security zones. Its basic policy can match source and destination addresses, ports, direction and connection state. Industrial models may also understand selected automation protocols, allowing rules based on commands or services rather than only IP and TCP/UDP details. Logging, alerting, network address translation, secure management and high-availability options are common design considerations.
The firewall is most visible at the enterprise-to-control boundary, where it can separate corporate services, an industrial DMZ and the control network. It is also useful between high-value cells, a safety system and a general control zone, or a vendor-access enclave and the production network. The device does not decide by itself which assets should trust one another; that decision comes from the zone-and-conduit design.
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What it can and cannot contain
- It can reject undocumented traffic, limit communication to named hosts and services, record sessions and provide a controlled point for remote-access brokers.
- It can reduce exposure at a boundary even when devices inside a zone are old or difficult to patch.
- It cannot compensate for a flat PLC network behind the boundary. Once an attacker or misconfigured workstation is inside that network, every reachable controller and remote-I/O path may still be exposed.
- An inline device that is not engineered for the plant’s power, environmental, failover and maintenance requirements can itself become an outage point.
What network segmentation does
Smaller trust domains
Segmentation divides the control environment into logical or physical areas with comparable security requirements. VLANs, Layer-3 routing, managed switches, physical cabling, security gateways and dedicated networks can all contribute. The objective is to limit which assets can communicate and to reduce the blast radius if a workstation, PLC or remote-I/O route is compromised.
A VLAN is not meaningful isolation by name alone. If inter-VLAN routing permits broad, implicit access, the networks remain one practical trust domain. Segmentation becomes enforceable when traffic crosses a controlled conduit with an allow list, logging, change control and a tested recovery procedure.
Why segmentation and firewalls are complementary
Siemens describes Ethernet networks as being segmented through “suppressors,” with firewalls, routers and Layer-3 switches among the common examples. One security zone can contain multiple network segments. That distinction matters: a zone is a risk-based grouping of assets, while a segment is an implementation detail. A firewall may enforce several conduits, and a single zone may use more than one segment for availability or process reasons.
MITRE ATT&CK for ICS mitigation M0930 recommends isolating critical systems, placing internet-facing services in a DMZ and restricting inter-zone access to required systems and services. The control is therefore architectural first and technological second: define the trust boundaries, then choose the least disruptive device that can enforce them.
How IEC 62443 defines the architecture
Zones, conduits and risk
IEC 62443-3-2:2020 requires the system under consideration to be defined, partitioned into zones and conduits, assessed for risk, assigned a target security level and documented with security requirements. A zone groups assets with comparable security requirements. A conduit is the controlled communication path between zones.
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IEC 62443-3-3 supplies system security requirements and capability security levels for the resulting control-system design. The standard does not say that every Purdue level must become a separate zone, nor does it prescribe one firewall model. The risk assessment determines whether two groups can share requirements and communications or need a boundary.
Why Purdue levels are not an automatic firewall map
Purdue levels are useful for discussing functions such as enterprise services, supervisory control, controllers and field devices, but a one-to-one mapping can create unnecessary complexity. A controller and its remote I/O may need continuous, deterministic communication and therefore belong to one cell/area zone. Two controllers at the same nominal level may have different consequences of compromise and require separate zones. Conversely, several device levels can be covered by one zone when their risks and required communications are comparable.
Questions the risk assessment must answer
- Which assets support the same process and have similar consequences if compromised?
- Which communications are required for control, safety, historian replication, patching, time synchronization and engineering?
- Which pathways cross a trust boundary, and what target security level applies to each side and conduit?
- What availability, deterministic-latency and fail-safe behavior must remain during a firewall failure, reboot or maintenance window?
- Which remote-access paths are permitted, who approves them and how are sessions recorded and revoked?
Where PLCs and remote I/O should be placed
Start with an asset and communication inventory
- List PLCs, remote-I/O adapters, HMIs, engineering workstations, safety systems, drives, historians, wireless gateways, network-management systems and every vendor-access path.
- Record each asset’s process role, required peers, protocols, maintenance dependencies, availability needs and consequence of compromise.
- Identify shared engineering stations and services that cross cell boundaries; they often need tighter controls than their physical location suggests.
Use cell/area zones for closely coupled control
Controllers and their associated remote I/O normally belong in a cell/area or control zone when they share security requirements and must communicate continuously. Keeping the controller and its I/O path in the same appropriately protected zone avoids inserting a filtering hop into a time-sensitive control loop merely to satisfy a diagram. The zone still needs a controlled conduit to HMIs, historians, engineering systems and other cells.
Separate assets when risk or trust differs
- Safety systems: Place a safety instrumented system in its own zone when its consequence, certification constraints or maintenance process differs from the basic control system.
- High-consequence cells: Give a process area its own boundary when compromise could affect production, personnel or the environment more severely than neighboring cells.
- Legacy controllers: Isolate devices that cannot be patched or authenticated from newer systems and restrict their permitted peers.
- Wireless gateways: Treat gateways as separate trust boundaries when they bridge mobile, contractor or less-controlled networks.
- Vendor-access services: Keep jump hosts and access brokers in a dedicated management or DMZ zone instead of placing a vendor VPN endpoint on a PLC subnet.
Reference zone layout
| Zone | Typical contents | Boundary purpose |
|---|---|---|
| Enterprise | Corporate users and business applications | No direct route to PLC or remote-I/O networks |
| Industrial DMZ | Jump hosts, controlled data brokers, update or replication services | Broker and inspect enterprise-to-control exchanges |
| Control | Supervisory servers, selected HMIs and control services | Limit access to cell, controller and process networks |
| Cell/area or controller | PLCs, drives and closely associated engineering or HMI functions | Contain lateral movement between production areas |
| Process I/O | Remote-I/O adapters and field-facing networks | Permit only required controller and maintenance traffic |
| Safety | Safety PLCs and related systems | Apply distinct trust, availability and change requirements |
This is a reference pattern, not a mandatory one-to-one mapping. Siemens’ architecture guidance likewise shows enterprise, DMZ, control, controller, process-I/O and safety zones while cautioning that device levels need not map directly to security zones.
How to build the enterprise-to-control boundary
Use an industrial DMZ rather than a direct route
Enterprise-to-control data exchange should pass through an industrial DMZ or an equivalent controlled conduit. Put brokered services there, not general-purpose access to PLC subnets. Typical documented flows include historian replication, approved patch or antivirus distribution, time synchronization and authorized engineering access. Each flow needs an owner, source, destination, service, direction, schedule and logging requirement.
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| Traffic purpose | Safer design question | Policy principle |
|---|---|---|
| Historian or reporting data | Can data be replicated outward without inbound control access? | Prefer one-way or brokered transfer where the product supports it |
| Patch and update services | Which repositories and maintenance windows are approved? | Allow named services for a defined schedule; deny general internet access |
| Time synchronization | Which time source is authoritative for control and security logs? | Permit only the required source and protocol across the conduit |
| Engineering access | Who needs access to which cell, for how long and with what approval? | Use a jump host or broker; do not expose the PLC subnet directly |
Keep rules explicit and reviewable
Begin with deny-by-default between zones, then add the minimum documented flows. Specify both directions where the protocol requires them; do not assume that allowing a session’s initial direction makes the return path safe in every device or protocol implementation. Log accepted and rejected connections at important boundaries, synchronize device clocks and review logs for unexpected peers, ports or schedules. Any temporary commissioning rule should have an owner, expiration date and removal check.
How to secure remote vendor access
- Terminate external access outside the control zone. Place the remote-access gateway or VPN termination in an enterprise or DMZ security zone, not on the PLC subnet.
- Require an approved jump host. The vendor connects to a hardened intermediary, then starts a second, controlled session to the authorized cell or engineering workstation.
- Use named identities and strong authentication. Give each person an individual account, enforce multi-factor authentication where supported, and avoid shared vendor credentials.
- Limit scope and time. Permit only the required host, protocol and cell, during an approved maintenance window. Disable the path automatically when the window ends.
- Record and supervise the session. Capture authentication, connection and administrative events; where operationally appropriate, record the jump-host session and require an operator to approve high-risk actions.
- Remove standing access. Revoke accounts, certificates, routes and temporary firewall rules when the work is complete, then verify that no direct vendor route remains.
The Australian Cyber Security Centre recommends network segmentation, jump boxes and secure firewall settings to block direct remote access to control systems. A vendor’s convenience is not a reason to bypass the conduit: direct VPN access to a PLC subnet collapses the intended least-privilege boundary.
Industrial firewall versus segmentation: practical comparison
| Decision axis | Perimeter firewall only | Fine-grained segmentation | Combined design |
|---|---|---|---|
| Blast-radius reduction | Strong at the plant edge; weak inside a flat control network | Reduces movement between cells and asset groups | Protects both the plant edge and internal boundaries |
| Protocol and application filtering | Centralized at a few boundaries | Depends on the routing and gateway controls used between segments | Places inspection where the risk and traffic pattern justify it |
| Latency and deterministic behavior | Usually avoids extra internal hops | Can add hops or policy processing that must be engineered | Keeps real-time loops local while filtering external and cell-to-cell flows |
| Remote-access control | Can broker access at the edge, but may still expose many internal hosts | Limits access to a cell or service | Uses a DMZ broker plus least-privilege internal rules |
| Availability and failover | Fewer devices, but a large single failure domain | More devices and policies to maintain | Requires deliberate redundancy, bypass and recovery design |
| Logging and investigation | Good edge visibility; limited detail after entry | More boundary events and better lateral-movement visibility | Correlates edge, DMZ and cell-level evidence |
| Engineering and lifecycle effort | Lower initial complexity, higher residual exposure | Higher design and testing effort | Highest effort, but normally the closest fit for a multi-cell risk model |
When to emphasize each control
Prioritize a perimeter firewall when
- The plant has no reliable enterprise-to-control boundary.
- Internet-facing or corporate services need to exchange data with control systems.
- Remote vendor access currently terminates on, or routes directly to, a control subnet.
- An industrial DMZ and jump-host pattern must be established quickly.
Prioritize finer segmentation when
- Several production cells share one flat PLC and remote-I/O network.
- Safety systems, legacy controllers or high-consequence processes have different requirements.
- Shared engineering workstations can reach more cells than their role requires.
- A compromise in one workstation or cell would provide an easy path to unrelated controllers.
Choose both when
A multi-cell plant has an enterprise boundary, several process areas, safety functions, legacy equipment or controlled vendor access. This is the normal IEC 62443-aligned pattern: risk-based zones, an industrial DMZ where appropriate, and a firewall, router, Layer-3 switch or security gateway enforcing each important conduit.
Implementation blueprint
- Define the system under consideration. Document plant boundaries, connected networks, wireless links, third-party services, maintenance paths and safety dependencies.
- Draw current traffic, not just topology. Record who initiates each connection, the required service, direction, frequency and failure consequence. Include undocumented support laptops and temporary cables.
- Perform the zone-and-conduit risk assessment. Group assets with comparable requirements; split groups when consequence, trust, availability or communication needs differ. Assign a target security level to each zone and conduit.
- Place PLCs and remote I/O deliberately. Keep tightly coupled controller/I/O traffic local to a cell or control zone, while separating safety, high-consequence, wireless and vendor-access functions when justified.
- Select enforcement points. Use a firewall, router, Layer-3 switch or security gateway according to required filtering, protocol support, throughput, environmental rating, latency, redundancy and maintenance constraints.
- Write the allow list. For every conduit, name source and destination assets, services, direction, schedule, owner, logging and expiration for temporary access. Deny everything not documented.
- Build the DMZ and remote-access path. Put brokers and jump hosts between enterprise and control networks; prohibit direct vendor VPN routes to PLC or remote-I/O subnets.
- Test failure behavior. Verify controller operation during device restart, link loss, policy reload, power failure and maintenance bypass. Confirm that a security control failure does not create an unsafe process state.
- Commission in stages. Observe traffic, pilot one noncritical cell, validate every required flow with operations, then migrate additional cells under change control.
- Operate the design. Review logs and rules, patch security devices through an approved process, recertify vendor accounts, test backups and update diagrams whenever assets or communications change.
Availability, deterministic behavior and recovery
Security policy is subordinate to safe, stable process operation. Before inserting an inline firewall or gateway into a controller-to-I/O path, determine whether its inspection, reboot or failover behavior is compatible with the control loop. Evaluate latency, jitter, connection-state handling, multicast or broadcast requirements, protocol timeouts and the effect of a blocked session.
For critical conduits, specify whether failure should be fail-open, fail-closed or transferred to a redundant device; the correct choice depends on process safety and the consequence of losing communications. Redundancy does not remove the need for a tested bypass or recovery plan: a pair of devices can share a bad rule or fail during a common maintenance event. Keep offline copies of configurations, documented rollback steps and a way to restore controller communications without weakening the entire architecture.
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Common failure modes and fixes
One firewall, one flat PLC network
Symptom: The enterprise edge is filtered, but every cell, HMI, engineering station and remote-I/O path can reach every other one. Fix: Create cell/area or other risk-based zones and enforce inter-zone conduits; do not rely on the perimeter device to contain an incident that has already crossed the edge.
VLANs with unrestricted routing
Symptom: Network diagrams show separate VLANs, yet the Layer-3 switch permits broad access between them. Fix: Apply explicit inter-VLAN policy, logging and change control, or use a security gateway where deeper inspection is required.
Overly broad allow rules
Symptom: A conduit permits an entire subnet, all ports or both directions “for compatibility.” Fix: Observe required traffic, document exact peers and services, narrow the rule, and test the application rather than preserving an unrestricted bridge.
Inline device causes a process outage
Symptom: A reboot, power loss, firmware update or stateful inspection issue interrupts control traffic. Fix: Validate industrial availability and environmental requirements, design redundancy or a controlled bypass, and test failure modes during a planned window.
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Symptom: A contractor receives broad network access after authenticating once. Fix: Terminate access in a DMZ, require a jump host, scope the destination and time window, record the session and remove access after the work.
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Purdue levels treated as mandatory zones
Symptom: The design contains many small boundaries that do not correspond to different risks or required communications. Fix: Revisit the IEC 62443 risk assessment and merge zones that genuinely share requirements, while separating assets whose consequence or trust differs.
Operating the architecture over its life
Change control and rule ownership
Assign an owner to every conduit and firewall rule. Require a reason, affected assets, security impact, test evidence, approval and rollback plan for changes. Review temporary commissioning and vendor rules for automatic expiry; a forgotten exception can silently undo the intended segmentation.
Monitoring and incident response
Collect firewall, router, switch, jump-host and authentication logs in a time-synchronized system that operations can access during an incident. Alert on new source addresses, unexpected protocols, denied traffic from engineering stations, activity outside maintenance windows and repeated attempts to cross cell boundaries. Logging should support both troubleshooting and investigation without placing a monitoring collector in the trusted control path.
Patchability and obsolescence
Security gateways, switches and firewalls become part of the control system’s lifecycle. Track firmware support, backup configurations, vulnerability advisories, spare hardware, licensing and the maintenance window required for upgrades. A device that cannot be patched or replaced safely can become a new high-risk asset, even if its original rule set was sound.
What the available evidence supports
IEC 62443 establishes the zone-and-conduit method and the associated security requirements; MITRE M0930 and the Australian Cyber Security Centre provide practical guidance on isolation, DMZs, jump boxes and secure firewall settings. These sources support a layered design, not a universal product choice or a guaranteed percentage reduction in incidents.
No authoritative numeric study directly comparing industrial firewalls with network segmentation for PLC and remote-I/O protection establishes a single winner. The appropriate decision depends on the plant’s risk assessment, protocols, availability constraints, existing topology and target security levels. Treat throughput, protocol support, redundancy and environmental fit as engineering requirements to verify for the specific installation.
A representative end-to-end design
In a multi-cell plant, corporate users reach an industrial DMZ, not the control network. The DMZ contains a hardened jump host and approved data-transfer services. A control-zone firewall permits only documented historian, patching, time and engineering flows. Each cell has its own routed segment or security gateway, with rules allowing the required HMI, controller and engineering communications. A safety system and a high-consequence cell have separate zones. PLCs and their associated remote I/O remain together where deterministic control traffic requires it, while process-I/O networks cannot initiate arbitrary sessions toward other cells. Vendor work is approved, time-limited and brokered through the jump host.
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This arrangement uses segmentation to define the trust boundaries and firewalls or Layer-3 enforcement to make those boundaries real. It also leaves operations with a documented way to test, monitor, fail over and recover the controls without resorting to permanent broad access.
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