An ESXi host generates the MAC address for a virtual machine adapter when the host is not connected to vCenter Server. Such addresses have a separate VMware OUI to avoid conflicts.
The ESXi host generates the MAC address for a virtual machine adapter in one of the following cases:
The host is not connected to vCenter Server.
The virtual machine configuration file does not contain the MAC address and information about the MAC address allocation type.
MAC Address Format
The host generates MAC addresses that consists of the VMware OUI
00:0C:29 and the last three octets in hexadecimal format of the virtual machine UUID. The virtual machine UUID is based on a hash calculated by using the UUID of the ESXi physical machine and the path to the configuration file (.vmx) of the virtual machine.
Preventing MAC Address Conflicts
All MAC addresses that have been assigned to network adapters of running and suspended virtual machines on a given physical machine are tracked for conflicts.
If you import a virtual machine with a host-generated MAC address from one vCenter Server to another, select the I Copied It option when you power on the virtual machine to regenerate the address and avoid potential conflicts in the target vCenter Server or between the vCenter Server systems.