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.