ICS Network Visibility: How to Monitor Industrial Networks 

Industrial Control System (ICS) networks operate under a fundamentally different set of priorities than enterprise IT networks. Uptime isn't just a performance metric in these environments – it's a safety and operational imperative. A conveyor line that stops unexpectedly, a valve that fails to respond, or a SCADA system that loses communication can trigger consequences ranging from lost production to physical harm. That reality shapes everything about how you approach network visibility in industrial settings.

The good news is that achieving comprehensive ICS network visibility without disrupting production is entirely possible. The key lies in choosing monitoring approaches that are inherently passive and non-intrusive. Network TAPs sit at the core of this approach, providing complete traffic copies to your security and monitoring tools without inserting any device into the live data path. Combined with intelligent network packet brokers that filter and distribute traffic efficiently, you can build a full-spectrum visibility architecture that your OT team can trust.

This guide covers everything you need to know: why ICS networks present unique visibility challenges, which monitoring approaches work in operational technology (OT) environments, and how to build an architecture that keeps both your security teams and your plant floor teams satisfied.

Why ICS Networks Are Different From IT Networks

The Uptime Imperative Changes Everything

In a traditional IT network, rebooting a server or taking a switch offline for maintenance is an inconvenience. In an ICS environment, the same action can halt production lines, disrupt critical processes, or – in industries like energy, water treatment, or chemical manufacturing – create genuine safety hazards. This means that any monitoring solution you introduce must be vetted against one overriding question: what happens to the production network if this device fails?

This is why standard enterprise monitoring approaches often can't be transferred directly into ICS environments. Agents installed on endpoints, inline security appliances, or tools that actively probe devices may all carry unacceptable risk when the devices being protected control physical processes.

Legacy Protocols and Older Hardware

ICS networks frequently run protocols designed for reliability and determinism rather than security. Modbus, DNP3, PROFINET, EtherNet/IP, and IEC 61850 are common examples. These protocols often lack encryption and authentication, which makes them inherently vulnerable but also means that monitoring tools need to understand them to provide useful analysis.

Compounding the challenge, ICS devices themselves are often legacy hardware running embedded operating systems that can't be patched, updated, or have agents installed. A Programmable Logic Controller (PLC) from 15 years ago may still be controlling a critical process today. You can't put an endpoint agent on it, which means your visibility strategy must be entirely passive and network-based.

Air Gaps Are Shrinking

Historically, many ICS networks operated in relative isolation from enterprise IT networks and the internet. That isolation has eroded significantly as organizations adopt Industrial Internet of Things (IIoT) connectivity, integrate OT data with business intelligence platforms, and enable remote maintenance capabilities. Each new connection point between the OT and IT domains introduces potential lateral movement paths for threats – and requires corresponding visibility coverage.

The Core Challenge: Monitoring Without Introducing Risk

The fundamental tension in ICS visibility is this: you need to see network traffic in order to detect threats and diagnose problems, but the tools you use to achieve that visibility must not themselves become a source of risk or disruption.

Why SPAN Ports Fall Short in OT Environments

In enterprise environments, many organizations rely on switch port analyzer (SPAN) ports to copy traffic to monitoring tools. SPAN ports have a number of limitations in any environment, but in ICS networks these limitations become particularly problematic.

SPAN ports are software-configured on managed switches, which means they can be accidentally misconfigured, overridden, or disabled during routine switch maintenance. More significantly, SPAN port mirroring consumes switch processing resources. On switches that are already handling deterministic, time-sensitive industrial protocols, additional load can introduce the latency and packet drops that ICS processes absolutely cannot tolerate.

Key SPAN port limitations in ICS environments include:

• Processing overhead: SPAN port mirroring uses switch CPU resources, introducing risk to time-sensitive OT traffic
• Packet loss under load: When switch buffers fill, SPAN traffic is dropped first, creating blind spots during the busiest – and most important – moments
• Configuration fragility: A switch firmware update, configuration change, or power cycle can reset or disable SPAN configurations without warning
• Limited port availability: Industrial switches often have fewer ports than enterprise equivalents, making SPAN port allocation difficult without impacting production connectivity
• No visibility into errors: SPAN ports don't forward errored frames, missing a class of traffic that is diagnostically important in OT environments

Why Inline Security Tools Carry Additional Risk

Inline security appliances – firewalls, intrusion prevention systems, and similar tools – sit directly in the data path between network segments. When they work correctly, they inspect and filter traffic in real time. When they fail, reboot for an update, or become overwhelmed by traffic, the result can be a complete traffic interruption.

In a data center, a brief interruption while a security appliance reboots is manageable. On a factory floor where a PLC is waiting for a heartbeat signal from a SCADA system, the same interruption can cause an emergency stop or a process fault. This is why inline tool deployment in ICS networks requires careful planning and, critically, bypass protection.

Passive Network TAPs: The Foundation of Safe ICS Visibility

The safest way to gain visibility into ICS network traffic is through passive network TAPs. Unlike SPAN ports or inline appliances, a passive TAP introduces no active component into the network path. It works by physically splitting the optical signal (for fiber links) or using transformer-based coupling (for copper), sending a complete, identical copy of all traffic to your monitoring tools while the original traffic continues uninterrupted.

How Passive Fiber TAPs Work in OT Networks

Passive fiber TAPs use optical splitters to divide the light signal on a fiber link. A portion of the light budget continues to the original destination, while the remainder is directed to your monitoring tool. There is no electronics, no IP address, no MAC address, and no configuration required. The TAP is completely invisible to the network.

This architecture delivers several properties that are critical in ICS environments:

• Zero latency impact: The optical split happens physically, not computationally. There is no processing delay added to the live network path
• No point of failure: Because there are no active electronics in the monitoring path, a monitoring tool failure or TAP power loss has no effect whatsoever on the live network
• 100% traffic capture: Every packet, including malformed frames and protocol errors, is delivered to your monitoring tool – providing the complete picture that ICS diagnostics require
• Invisible to attackers: With no IP or MAC address, the TAP cannot be targeted, scanned, or exploited by threat actors on the network
• Always-on operation: Even during power outages, passive fiber TAPs continue to pass traffic on the live link, because they require no power to do so

Active Ethernet TAPs for Copper ICS Links

Many industrial networks still use copper Ethernet for device connectivity, particularly at the field device level where PLCs, remote terminal units (RTUs), and human-machine interfaces (HMIs) connect. Ethernet TAPs provide the same fundamental monitoring capability for copper links.

Active Ethernet TAPs use transformer-based hardware coupling to copy traffic without interrupting the live connection. They include fail-safe protection, meaning that if the TAP loses power, the copper link automatically passes through uninterrupted. This is the "fail-open" property that OT teams require: the monitoring infrastructure must never be the reason a production device loses connectivity.

Building an ICS Visibility Architecture

Once you've established passive TAPs at your key network monitoring points, the next challenge is managing the traffic those TAPs generate and getting the right data to the right tools efficiently. This is where network packet brokers play a critical role.

Aggregation: Consolidating Traffic Streams

A typical ICS network has multiple network segments: a process control network connecting PLCs and controllers, a supervisory network running SCADA systems, a historian network, and potentially an industrial DMZ separating OT from IT. Each segment may have multiple links that require monitoring.

Without aggregation, you'd need a dedicated monitoring tool for every TAP, which quickly becomes impractical. A packet broker aggregates traffic from multiple TAPs into consolidated streams, allowing a single tool to monitor multiple network segments simultaneously.

Filtering: Delivering Relevant Traffic to Each Tool

Not every tool needs to see every packet. Your ICS-specific intrusion detection system (IDS) needs to analyze industrial protocol traffic. Your anomaly detection platform needs to see device communications to build behavioral baselines. Your forensic capture appliance may only need to record traffic during specific time windows or incident investigations.

Packet brokers apply filtering rules that direct specific traffic types to the tools designed to analyze them. Common filtering criteria in ICS visibility deployments include:

• Protocol-based filtering: Route all Modbus, DNP3, or EtherNet/IP traffic to your OT-aware IDS
• IP segment filtering: Send traffic from specific network ranges to dedicated tools
• Port-based filtering: Direct traffic on specific application ports to relevant monitoring platforms
• VLAN-based filtering: Separate traffic by network zone and distribute to zone-specific tools
• Time-based policies: Capture and store traffic only during specific maintenance windows or after anomaly alerts

Load Balancing: Distributing Traffic Across Tool Clusters

At higher traffic volumes, a single monitoring tool may not be able to process every packet at line rate. Packet brokers support load balancing across multiple instances of the same tool type, ensuring session-aware distribution that keeps related traffic flows together. This is particularly important for ICS anomaly detection tools that build device behavior profiles over time – you need consistent session routing to avoid fragmenting the behavioral picture.

Protecting Inline Tools with Bypass TAPs

Some security functions simply can't be performed out-of-band. Deep packet inspection for certain threat types, or active blocking of known malicious commands, requires inline placement. If you need to deploy inline security tools in an ICS environment, bypass TAPs are what make that deployment safe.

How Bypass Protection Works

A bypass TAP sits around an inline security appliance. It continuously sends heartbeat signals through the appliance. If the appliance stops responding – due to a software fault, reboot, power failure, or maintenance activity – the bypass TAP automatically redirects traffic around the failed appliance in milliseconds. The production network never loses connectivity.

When the appliance recovers, the bypass TAP detects the restored heartbeat and gradually reintroduces live traffic, allowing the appliance to re-synchronize its state before handling full production loads.

Key bypass TAP capabilities for ICS deployments include:

• Automatic failover: Millisecond-level detection and bypass of inline tool failures
• Maintenance windows without downtime: Take inline tools offline for updates without interrupting traffic
• Heartbeat monitoring: Continuous health checking ensures failures are detected before they affect production
• Gradual traffic restoration: Controlled reintroduction of traffic after appliance recovery
• Hot-swap power supplies: Dual redundant power feeds prevent TAP-level single points of failure

Key Monitoring Points in an ICS Network

Knowing where to place TAPs is as important as understanding the technology itself. The right monitoring points give you comprehensive visibility without requiring a TAP on every single link.

Network Zones and Boundaries

The most valuable monitoring points in an ICS network are typically at zone boundaries, where traffic crosses from one security zone to another. These locations let you see all cross-zone communications, which is where both legitimate operational traffic and potential lateral movement threats will appear.

Priority monitoring locations include:

1. The industrial DMZ perimeter: The boundary between OT and IT networks captures all communications between the two domains
2. SCADA server connections: All traffic to and from SCADA servers shows both legitimate supervisory communications and potential command injection attempts
3. Historian server links: Data flows to historian systems reveal what information is being collected and can surface abnormal data exfiltration
4. Remote access gateways: VPN and remote access points are common attack entry vectors that benefit from full traffic capture
5. PLC and RTU uplinks: Controller-level visibility catches command and response traffic at the device level

Within-Zone Monitoring

Within individual network zones, monitoring key device uplinks provides device-level visibility. Not every device requires dedicated monitoring, but critical controllers, primary SCADA servers, and safety system connections typically justify individual TAP coverage.

OT-Aware Security Tools: What to Connect to Your Visibility Infrastructure

Your TAPs and packet brokers create the visibility layer, but the analysis and detection work happens in the tools you connect to that infrastructure. ICS environments require tools that understand industrial protocols natively.

Industrial Protocol Analysis

Standard IT network analysis tools don't understand Modbus function codes, DNP3 application layer messages, or PROFINET cyclic data. OT-aware network analysis platforms decode these protocols and can detect anomalies like unexpected function codes, commands from unauthorized source addresses, or changes in polling frequency that might indicate reconnaissance activity.

Behavioral Anomaly Detection

Because many ICS devices communicate on regular, predictable schedules with consistent message patterns, anomaly detection is highly effective in OT environments. A PLC that suddenly starts communicating with an IP address it has never contacted before, or an HMI that begins issuing commands outside its normal operational parameters, represents a meaningful deviation worth investigating.

Passive Asset Discovery

One of the most immediate benefits of ICS network visibility is automatic asset inventory. Many OT environments lack up-to-date records of every connected device. Passive traffic analysis can identify devices by their communication patterns, protocols, and source addresses, building a real-time asset register without sending a single probe packet to any device.

Benefits of passive asset discovery include:

• No device impact: Discovery happens by observing traffic, not by scanning devices
• Continuous updates: New devices appear in the inventory as soon as they communicate
• Protocol identification: Tools identify device types and likely manufacturers from protocol usage patterns
• Change detection: Newly appeared devices or unexpected communication patterns trigger alerts automatically

Compliance Considerations for ICS Network Monitoring

ICS environments in critical infrastructure sectors face regulatory requirements that create specific visibility obligations. NERC CIP standards for the electric sector, IEC 62443 for industrial automation, and sector-specific guidance from the Cybersecurity and Infrastructure Security Agency (CISA) all emphasize the importance of network monitoring and anomaly detection.

What Regulators Typically Require

While specific requirements vary by standard and sector, most ICS security frameworks share common monitoring expectations:

• Traffic logging at zone boundaries: Records of communications crossing security perimeters
• Anomaly detection capability: Systems capable of identifying deviations from established baselines
• Incident investigation support: Forensic traffic capture capabilities that support post-incident analysis
• Change management visibility: Records of network configuration changes and new device appearances

Passive TAPs Support Compliance Without Operational Risk

The compliance case for passive TAP-based visibility architecture is straightforward: you can demonstrate comprehensive monitoring coverage across all required network zones without accepting the operational risk of inline tools or the reliability limitations of SPAN-based approaches. Audit trails, traffic logs, and forensic captures can all be delivered from your visibility infrastructure to compliance reporting platforms.

Practical Deployment Considerations

Moving from concept to deployment requires addressing several practical questions specific to ICS environments.

Change Management in OT Environments

Any modification to an ICS network requires careful change management. Even deploying a passive TAP, which carries no operational risk, needs to go through your organization's OT change management process. This typically involves:

1. Documenting the proposed change and its justification
2. Review and approval by OT engineering and operations
3. Scheduling maintenance windows for physical installation
4. Testing and verification before returning links to full production status
5. Documentation updates to network diagrams and asset records

Working With OT and IT Teams Together

Successful ICS visibility projects almost always involve close collaboration between OT engineering teams and IT security teams. OT teams understand the operational constraints, the sensitivity of specific devices, and the change management processes. IT security teams understand threat landscapes, monitoring tool capabilities, and visibility architecture principles.

The most common source of friction is timeline: OT teams often have very limited maintenance windows and long change approval cycles. Building trust by starting with out-of-band passive monitoring (which carries zero operational risk) before proposing any inline security tools is a consistently effective approach.

Choosing the Right TAP Form Factor

ICS environments often have different physical infrastructure than data centers. Industrial switches may be DIN-rail mounted in compact enclosures, running at temperatures and humidity levels outside typical data center specifications. When selecting TAP hardware for OT deployments, consider:

• Environmental ratings: Operating temperature range, humidity tolerance, and vibration resistance
• Physical form factor: Rack-mount TAPs versus panel-mount or DIN-rail-compatible options
• Power supply redundancy: Dual power inputs provide resilience appropriate for critical environments
• Fiber versus copper: Match the TAP type to the physical media in use on each link

Frequently Asked Questions

Will Installing a Passive TAP Affect My ICS Network Performance?

No. Passive fiber TAPs work by physically splitting the optical signal and require no processing, adding zero latency to the live network path. Active Ethernet TAPs for copper links use hardware coupling and include fail-safe designs, so a TAP power failure causes no network interruption. The monitoring traffic flows entirely out-of-band and cannot affect live network performance.

Can I Monitor ICS Network Traffic Without Touching the Production Devices?

Yes. The TAP-based visibility approach specifically avoids any interaction with production devices. TAPs install on the physical links between devices, not on the devices themselves. No agents, no configuration changes on PLCs or SCADA servers, and no active probing of any device is required.

How Do I Monitor Legacy Serial Links in ICS Environments?

Many older ICS installations still use serial connections for device communication. Serial-to-Ethernet protocol converters are often already deployed to integrate legacy serial devices with modern network infrastructure. Monitoring the Ethernet side of those converters with standard network TAPs provides visibility into the underlying serial protocol traffic.

What's the Difference Between IT Security Monitoring and ICS Security Monitoring?

The fundamental difference is in protocol understanding and operational priorities. IT monitoring tools are designed for standard TCP/IP protocols and enterprise application traffic. ICS monitoring requires understanding industrial protocols like Modbus, DNP3, and PROFINET. Additionally, ICS monitoring must prioritize operational availability above all else – a monitoring approach that could potentially disrupt a production process is unacceptable regardless of its security value.

How Much Traffic Do ICS Networks Typically Generate?

ICS network traffic volumes are generally much lower than enterprise IT networks. Industrial control traffic is often highly predictable: regular polling cycles, deterministic control messages, and periodic status updates. This low, predictable traffic volume makes ICS environments particularly well suited to comprehensive packet capture and full-session recording, which would be impractical at IT network speeds.

How Network Critical Can Help

Building ICS network visibility infrastructure requires hardware specifically designed to deliver complete traffic access without introducing any risk to operational continuity. Network Critical has provided network visibility solutions to organizations in critical infrastructure, energy, manufacturing, and government sectors since 1997, with deployments where network reliability is an absolute requirement.

Our passive fiber TAPs deliver 100% traffic capture across fiber links from 1Gbps to 100Gbps with zero power dependency and no impact whatsoever on live network traffic. For copper Ethernet links running to PLCs, HMIs, and field devices, our Ethernet TAPs include fail-safe protection that ensures the production link passes through uninterrupted even if the TAP loses power.

The SmartNA-XL combines TAP and packet broker functionality in a compact modular chassis, supporting 1G/10G/40G speeds with advanced filtering, aggregation, and load balancing through the intuitive Drag-n-Vu management interface. For environments where inline security tools are required, our bypass TAPs provide automatic failover protection that keeps production traffic flowing regardless of what happens to the inline appliance.

Whether you're building initial visibility into a previously unmonitored OT network, extending coverage to new network zones, or supporting compliance requirements under NERC CIP, IEC 62443, or sector-specific frameworks, our team can help you design an architecture that delivers comprehensive monitoring without compromise to your production environment.