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.