Hyper-V R2: MAC Address Management
Hyper-V has always allowed you to configure virtual network adapters where the MAC address was generated dynamically. These dynamic MAC address are selected from a random pool of MAC addresses. With Windows Server 2008 R2 we have added a new section to the Virtual Network Manager that allows you to configure the range of MAC addresses that Hyper-V will use for dynamic MAC address generation:
This is very handy if you need to ensure that multiple physical servers in your environment are not using overlapping MAC address ranges.