In modern computing we often don’t have to think about MAC addresses thanks to the ARP protocol. These MAC addresses are assigned by hardware manufacturers and are hardcoded into network interface cards to be globally unique. The concern arises in virtual networking in that the virtual machines we are using do not have hardware manufacturer assigned MAC addresses.

 Virtual Machines can use the same NIC as the host that connects to a network but they cannot share the same MAC address. This would cause confusion in the ARP tables and result in communication not getting to where it needed to go. Hyper-V addresses this issue by allowing you to assign a dynamic MAC address or setting a static one. It is important that whatever option you do choose you remain consistent and ensure that no duplicate MAC addresses are on the network. If you do attempt to start a VM using a MAC address that’s already in use, you will receive an error message stating “Attempt to access invalid address.”