VMware Organizationally Unique Identifier (OUI) allocation uses the default OUI 00:50:56 as the first three bytes of the MAC address generated for each virtual network. The MAC address-generation algorithm produces the rest of the MAC address. The algorithm guarantees unique MAC addresses within a virtual machine and attempts to provide unique MAC addresses across virtual machines.

VMware OUI allocation is set as the default MAC address generation schema for virtual machines.

The VMware Universally Unique Identifier (UUID) generates MAC addresses that are checked for conflicts. The generated MAC addresses are created by using three parts: the VMware OUI, the SMBIOS UUID for the physical ESXi machine, and a hash based on the name of the entity that the MAC address is being generated for. All MAC addresses that have been assigned to network adapters 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 when a virtual machine is powered on again, it can acquire a different MAC address. This acquisition is caused by a conflict with a virtual machine that was powered on when this virtual machine was powered off.

However, if you reconfigure the powered off virtual machine's vNIC, for example by updating the vNIC MAC address allocation type, or specifying a static MAC address, vCenter Server resolves any MAC address conflict before the vNIC reconfiguration takes effect.