Yesterday I found an interesting bug in Ubuntu Server 10.04 LTS (kernel 2.6.32-33-generic-pae) on Hyper-V. I set up static address for Legacy Interface, but I could not even ping the gateway or some host at the same network. Then i tried to use DHCP, the system get address and other network settings from DHCP server during boot, but i saw the same effect - "Destination unreachable". I had already few Ubuntu server on Hyper-V and network works good (but only Legacy, with Synthetic system hangs). I tried to ask google about this problem, but found no solutions, so i started search the difference. After spending a bunch of time i found it - "Number of processors". If you set it value bigger than 1, something breaks at ubuntu guest and network stop working.