An unknown device with a strange mac-adress in my FritzBox

Hello Iam a owner of the “Brume W” (GL-MV1000W),

I use the Brume over the Brume-WAN-Port, with a lan-cable-connection, behind my Fritz!Box.
Again: One end of the lan-cable plugs into the WAN-port of my Brume and the other end of the lan-cable
plugs into a lan-port of my Fritz!Box.
In my Fritz!Box I can see the mac-adress of my Brumme.
I will not post the real mac-adress in this posting …

I use an example:
The mac-adress in this example is:


My problem is:
After all the work to install my Brume in my network I found a unknown device, with the internal IP …
and the mac-adress of this unknown device is

The only difference between my Brume-mac-adress and the mac-adress from the “unknown device” is the last digit ( 4 and 5 ).

The DHCP-Server of my FritzBox creates only IPs, that starts with 20 (example

What is this for a “device”?
A virtual machine on my Brume or something like this???

I dont know!

Iam very lucky when somebody can help!

I hope Iam right here in this Board!

Greetings from Germany and sorry for my bad english!