Once passive discovery mechanisms are enabled, organizations frequently identify additional assets. In some cases, the difference amounts to several or even more than ten percent compared with the official inventory. These assets may include undocumented printers, laptops connected “just for a moment,” test devices, private computers, or equipment used by external implementation teams.
All of these fall under the broader category of shadow IT — assets operating within the corporate network without the organization’s official knowledge, control, or documentation.
In the context of NIS2, asset awareness and visibility into network activity form the basis of effective risk management. The question is therefore not only whether to maintain an inventory, but also how to do it continuously and safely.
There are two main approaches: active and passive inventory.
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Two models: active and passive inventory
Active inventory is the traditional approach based on periodic network scanning. A tool sends requests to selected addresses or devices, checks which ones respond, and builds an asset list based on the results.
This approach is intuitive and can quickly provide a snapshot of a specific subnet. It can also actively verify host availability, open ports, running services, and other technical details.
Passive inventory works differently. It does not send requests to devices. Instead, it observes current and historical network traffic and builds an asset list based on actual communication within the monitored environment.
When a device communicates through a part of the network covered by monitoring, its activity may be recorded in historical data and remain visible even after the device has been disconnected.
Both models provide valuable information, but they address slightly different needs. Active scanning shows what is available at the moment the scan is performed. Passive discovery shows what has actually communicated within the network over a longer period.
In large and dynamic environments, that difference matters.
A limitation of active scanning: additional network traffic
Network scanning is not entirely neutral to the environment. By definition, it requires sending a certain number of requests that devices must process and respond to.
The impact of a scan depends on its configuration, scope, intensity, and the number of assets checked simultaneously. A properly configured scan does not necessarily cause problems. However, in large or particularly sensitive environments, an overly aggressive scan may generate a significant number of additional connections within a short period.
In a recent webinar about NIS2, Sycope expert Piotr Kałuża shared real implementation examples where teams preparing compliance documentation ran intensive network scans and unintentionally put additional load on network devices.
As a result, the network operations team received alerts related to performance degradation or service availability. From an operational perspective, this traffic could resemble unusual, large-scale activity originating from inside the organization.
This does not mean that every active scan leads to overload. It does, however, show that scanning must be carefully planned, limited, and adjusted to the capabilities of the environment.
Passive inventory does not generate active traffic directed at hosts. It observes communication that is already taking place in the network, so it does not require every device to be actively queried.
A limitation of periodic scanning: visibility at selected moments
The second limitation is related to time.
Periodic scanning shows which devices were visible when the scan was performed. If an asset appears and disappears between two scans, it may never be recorded.
Piotr also provided two specific examples.
The first is a private computer connected during a night shift, for example to access the internet. By the morning, the device is no longer in the network, so a scan scheduled for later will not detect it.
The second example is an external implementation engineer who connects to the organization, opens a support tunnel, downloads updates, and finishes the work before the next scheduled scan.
In both cases, the devices may remain invisible to a traditional periodic inventory process. From a security perspective, however, these are exactly the assets an organization should know about. They may not comply with internal policies, may lack current security updates, or may operate outside the control of the IT team.
Passive discovery can record such devices, provided that their communication passes through a monitored part of the network and is included in the analyzed data.
By using historical traffic data, an asset may remain visible in the inventory even when it is no longer connected at the time of analysis.
The advantage of passive discovery: continuous visibility without active polling
The main advantage of passive inventory is that it can provide a more continuous view of the environment without actively polling devices.
It makes it possible to identify assets that have actually communicated within the monitored part of the network, including devices connected only briefly or operating between scheduled scans.
These are two separate benefits.
The first concerns visibility over time. Historical data analysis increases the likelihood of identifying temporary, short-lived, and undocumented assets.
The second is operational. Passive discovery does not generate scanning traffic or require a series of requests to be sent to every host.
This does not mean that passive inventory is completely independent of the network architecture. Its effectiveness depends on factors such as monitoring coverage, data export configuration, visibility across individual network segments, retention, and potential traffic sampling.
It will also not detect a device that remains completely inactive or whose communication does not pass through a monitored point.
For this reason, passive and active methods can complement each other. Passive inventory works well as a continuous observation mechanism, while active scanning can provide additional information about host availability, open ports, and running services.
Asset discovery in Sycope — passive and automatic
In Sycope, the passive inventory model is available in the Asset Discovery module.
The system builds an asset list based on observed network traffic. There is no need to initiate every new scan manually — the view of the environment is updated as new devices and communication become visible in the data.
In practice, one of the first steps after enabling the module may be to compare the resulting asset list with the organization’s existing documentation.
This is often where the discoveries begin: devices that are currently operating, or have previously operated, in the network but were never included in the official inventory.
These may include both permanent assets and devices connected only temporarily.
What this means for environment inventory
Effective monitoring and risk analysis are only meaningful when they cover as complete a view of the environment as possible, rather than only the assets listed in the documentation.
Passive inventory can therefore serve as one of the foundations of real network visibility. It allows analysis to be based not only on a declared list of devices, but also on data showing what has actually communicated within the environment.
Active scanning remains a useful tool. It makes it possible to quickly check a specific subnet, confirm device availability, or obtain information about services. However, when used as the only inventory mechanism, it may miss assets operating between scheduled scans and requires controlled generation of additional traffic.
The most practical approach does not necessarily involve completely abandoning active scanning. The key is not to base the entire view of the environment solely on periodic snapshots.
Shadow IT is not an abstract concept taken from a cybersecurity presentation. It may be a laptop connected to the network last night, a test device running for a few hours, or an external engineer’s computer.
A periodic scan may never detect them. Historical network traffic may still leave a trace.
