Build a network inventory before changing security settings

Start with observation

A useful network inventory answers three questions: what is connected, who owns it and what business function it serves. A list of IP addresses alone cannot answer them. Addresses change, device names can be vague and a printer may depend on a computer that nobody remembered to document.

Begin with existing router and operating-system views. You do not need a subscription scanner, and this exercise does not authorise scanning someone else's wireless network or a supplier's infrastructure.

Build a small private register

1. Record the router model, supported firmware, local subnet and how authorised administrators access it. Keep passwords in the approved vault rather than in the inventory.
2. Open the router's connected-client or DHCP view. Match entries to known computers, phones, printers and equipment using device settings and physical labels. A MAC address is a clue, not reliable proof of identity.
3. Record the device owner, purpose, connection type and whether it needs access to other local devices. Note reserved addresses separately from ordinary DHCP leases.
4. Identify unknown entries without immediately blocking them. Ask staff about recently added equipment and compare the observation time with expected use. An offline device may not appear in the current view.

Verify a dependency

Choose a harmless workflow, such as printing a test page or opening an internal status page. Record which devices participate and which address is used. This gives you a baseline before moving equipment to a guest network or changing a firewall rule.

Check a second time after normal business activity. A payment terminal, meeting-room system or backup destination may only appear during particular operations. Document the limits of your observation rather than claiming the first list is complete.

Handle unknown devices proportionately

For an unexplained client, review router information and consult the authorised network owner. Do not publish full device identifiers or guest information. If immediate containment is required, record the change and preserve a route to recover legitimate access.

Keep the inventory useful

Update it when adding or retiring equipment, changing internet providers or investigating an incident. Link each security change to a known device or purpose. An accurate ten-device register is more useful than an enormous scan export with no owner, no date and no explanation of what the equipment actually does.