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 subject here). Hyper-V R2 (download) has provided the ability to manage the address pool directly in the Virtual Network Manager. To manage the global MAC address pool, open up Virtual Network Manager and select the MAC Address Range option under Global Network Settings. From here you can modify the MAC address pool available on the managed Hyper-V host.