MAC address

I have been looking at my father-in-law's network and there is a device connected that we cannot identify ...

The MAC address starts 46: I have checked various web sites to get and find the device, but nothing. Is there a full database that holds all MAC allocations.
I really need to find the device. ... Just have an uneasy feeling about it.

My PC, wife's PC, my phone, her phone, Apple Mac, alarm, wireless extender, Apple TV, are all accounted for.

Tried to ping it at xxx.xx.x.133 and the responses were: No response from xxx.xx.x.134 or 84 which was a bit odd.

The IEEE registration authority is where the MAC addresses are aligned to companies, their lookup system is heavily restricted. You're after an OUI lookup and Wireshark is a place to start:
https://www.wireshark.org/tools/oui-lookup

You will need the whole MAC to do a lookup, often the first 4 or 5 sections are the company, and can be either the hardware maker or the assember. e.g. Apple uses broadcom for WiFi and Ethernet chips but has their own MAC allocation, other vendors using Broadcom Ethernet cards might show Broadcom as the vendor.

Thanks ... the full MAC is 42:9a:34:d2:80:58 and nothing appears anywhere and I've tried the various formats.

You could try running Nmap (the Zenmap GUI) against the IP to see what that reveals.

If the locally-administered address (LAA) bit is set (i.e. the first octet is x2, x6, xA or xE) the address is made up so won't appear in any registration databases, see https://en.wikipedia.org/wiki/MAC_address#Universal_...

Most commonly generated by network privacy functions in newer mobile device OS.

In reply to a post by tdw42:

Most commonly generated by network privacy functions in newer mobile device OS.

Also available in macOS and Windows11 (may even be in Win10). Defaults to on with iOS/iPadOS and shows "privacy alert" when disabled, even when you want to statically assign IP to MAC...

The worry is that someone has secreted a device in the house - possibly using the WPS set up on the router/WAP. And I want ot find out exactly what it is.

A possible ……

Apple devices may have private wifi enabled

https://support.apple.com/en-gb/HT211227#:~:text=Abo...

Thanks, tried that ...

The IP brings back the same MAC and reports no OS match for host ... nothing in te data returned points to a known device.

In reply to a post by MHC:

The worry is that someone has secreted a device in the house - possibly using the WPS set up on the router/WAP. And I want ot find out exactly what it is.

Change the WiFi key on the main router, set up a second router with the old SSID and WiFi key and see if anything connects.

Win11:
https://support.microsoft.com/en-us/windows/how-to-u...