How do I build an OT asset inventory without active scanning?

Combine three passive sources: what your documents say, what your engineering tools know and what the network shows. Start with the drawings, the engineering project files, which list every controller, drive and remote I/O they configure, and the supplier documentation of each machine. Then walk the plant: cabinets, panels, remote sites. Finally, add what the traffic shows: a passive sensor on a mirror port sees every device that communicates on that segment, including the ones nobody wrote down. Record for each asset its address, function, owner, firmware version and whether it can be patched. The NetSenX network map, on every plan, lists devices seen on monitored segments with the names they announce and first and last seen, and from the Pro plan NetSenX reads Modbus/TCP, DNP3, BACnet/IP, EtherNet/IP and S7comm. It never probes OTOT: operational technology. The computers that run machines, production lines, building systems and utilities. devices, so it does not query firmware versions; those come from your engineering tools.

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