Server 2008 R2 VM Guest on Hyper-V: vNIC defaults to Public Network instead of Domain

This morning, after a reboot, one of my 2k8VMs decided that it was no longer on the domain and couldn't find any domain controllers, therefore it defaulted to a Public Network for the NIC.

This had the unfortunate effect of rejecting all traffic sent its way.

I did some quick searching, but it appears that my Google-Foo is not strong at 8am before coffee and I came up blank.

I managed to resolve this by logging into the virtual console through Hyper-V manager and change the settings on the NIC to enable DHCP and grab an IP address. This immediately saw everything again and happily reported to be on the domain with internet access.

I then reset the NIC to a static IP (the same details as before), which it held and maintained its ability to talk to the domain.

Why? I dont know... anyone?