On ESXi hosts 5.1 and later, you can use prefix-based allocation to specify an OUI other than the default one 00:50:56 by VMware, or to introduce Locally Administered MAC Addresses (LAA) for a larger address space.

Prefix-based MAC address allocation overcomes the limits of the default VMware allocation to provide unique addresses in larger scale deployments. Introducing an LAA prefix leads to a very large MAC address space (2 to the power of 46) instead of an universally unique address OUI which can give only 16 million MAC addresses.

Verify that the prefixes that you provide for different vCenter Server instances in the same network are unique. vCenter Server relies on the prefixes to avoid MAC address duplication issues. See the vSphere Troubleshooting documentation.