How can I see unknown devices on the company network?

Start with what you already have: the DHCP leases on your router or server, the MAC address tables on your switches and the client list of the Wi-Fi controller. Compare them with your inventory and investigate anything unexplained; the first half of a MAC address identifies the manufacturer, which often tells you whether a device is a printer, a phone or a small computer. Port security or 802.1X stops unknown devices from joining in the first place. The NetSenX network map, on every plan, shows the hosts seen on monitored segments and who they talk to, so a new device that starts talking stands out. A device that stays silent, or sits on a segment you do not monitor, does not appear there.

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