I reorganized a 48U cabinet I have, moved around a couple of servers and a switch plus cleaned up a bit of cabling. That was all.

When I started the switch and the server again suddenly I see 192.168.137.* addresses called mshome.net on my Mikrotik Router.
I have no network called that or in that range. I have no computers assigned to such a network group or named that way.

Found older data on the web saying that:

MShome.net is normal. I happens when you configure Internet Connection Share (ICS) on a computer. The ICS host requires a DNS name. If the computer has not been assigned a DNS domain name then MShome.net is automatically applied to the host computer.

I have nothing of the kind, yet how would I go about checking this?

I am aware that you can get automated private non connected addresses at 169.254.*.* but none of that happened so I am rather puzzled about where that 192.168.137.* came from. I got rid of it by deleting the entry in the routers leases and resetting the relevant interface.

Anyone have any ideas...?

On each PC open Settings > Network & Internet, select Change adapter settings. Right-click the wired network connection (often just called Ethernet), select Properties. On the Sharing tab ensure that Allow other users to connect through this computer's Internet connection is not ticked, it it is untick it and slap the user.

I did. Sharing is not enabled. On W11, my main desktop, sharing is not even a visible option in Adapter Properties.

Any other PCs connected? I'm not sure that the offending MAC address will show up in the Mikrotik ARP table as the Mikrotik is not using that subnet, you might have to use the packet sniffer (or Wireshark on a PC), to spot it in broadcasts.

Yes, there are like 5 other Windows PC's connected, but I did get rid of it and the culprit was my HPE Proliant Windows Server, one of its two "normal" NIC's, it has a third for ILO.

I am more curious as to how the Router could give out an IP it was not even configured to handle.

You can have multiple subnets (layer 3) on an ethernet network (layer 2). You could have one group of devices using 192.168.1.x/24 and another group using 192.168.2.x/24, the only thing you can't do is have non-static DHCP for both subnets.

When you say I see 192.168.137.* addresses called mshome.net on my Mikrotik Router, where exactly did you see them?

I am aware I can have multiple lans/dhcp's and all that, but the thing is I do not have them.

it was in the DHCP leases list... and also in ARP list.

I found it a few days ago. Turns out it was an ILO setting on one of my two HPE servers. The ILO IP was ok, but I did not see any setting for mshome.net. It was entered in the BIOS settings of the server.