Upon virtual machine creation, ESXi or vCenter Server systems assign each virtual network interface its own unique MAC address. The first three bytes of the MAC address that is generated for each virtual network adapter consists of a manufacturer-specific Organizationally Unique Identifier (OUI). The MAC address-generation algorithm produces the other three bytes. vSphere generates MAC addresses that are checked for conflicts. After the MAC address has been generated, it does not change unless the virtual machine is moved to a different location.
All MAC addresses that have been assigned to virtual network interfaces of running and suspended virtual machines on a given physical machine are tracked. The MAC address of a powered off virtual machine is not checked against those of running or suspended virtual machines. It is possible that a virtual machine acquires a different MAC address after a move.
The ESXi Configuration Guide discusses virtual machine MAC addresses in detail.
