Hyper-V automatically creates a MAC address pool for virtual machines usage when it is installed. Hyper-V RTM had no option in the UI for managing the MAC address pool and you had to edit the registry to manage the pool (see a detailed article on that
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When you install Hyper-V R2 role now it reserves a single wired NIC in the machine for management of the parent partition. If you have only a single NIC in your server, you will not be able you select a NIC to create an external network when you are installing
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