A checkbox is introduced in the Hyper-V UI for Windows 7 and Windows Server 2008 R2 which determines whether a virtual NIC is created in the parent partition for External Virtual Networks. If you follow best practice of having at least two physical NICs, one for use by the parent partition (aka Management Operating System), and one or more for use by virtual machines. But using the “v1” UI, there’s no option to not have the parent virtual NIC created. Two common ways to resolve this are to disable the virtual NIC it in “Network Connections” on the control panel, or unbinding all protocols from the virtual NIC. The disabling option tends (in my experience) to be the more common choice of the two.

Full Article: Hyper-V Blog