MAC addresses are used in the Layer 2 (Data Link Layer) of the network protocol stack to transmit frames to a recipient. In vSphere, vCenter Server generates MAC addresses for virtual machine adapters and VMkernel adapters, or you can assign addresses manually.
Each network adapter manufacturer is assigned a unique three-byte prefix called an Organizationally Unique Identifier (OUI), which it can use to generate unique MAC addresses.
VMware supports several address allocation mechanisms, each of them with a separate OUI:
Generated MAC addresses
Assigned by vCenter Server
Assigned by the ESXi host
Manually set MAC addresses
Generated for legacy virtual machines, but no longer used with ESXi
If you reconfigure the network adapter of a powered off virtual machine, for example by changing the automatic MAC address allocation type, or setting a static MAC address, vCenter Server resolves any MAC address conflict before the adapter reconfiguration takes effect.