During a VMware Install, Configure Manage class I had two students each with seperate vCenters complaining about their virtual machines were receiving the same IP address from the DHCP server. At first I thought there was some weird problem with my DHCP server but that was not the case. We checked the VMware generated MAC addresses for the problem VM’s and guess what… they had the same MAC addresses. That pretty much told us why they were having IP conflicts.
At that time I was not aware how vCenter generates is MAC addresses but after asking google and kb.vmware.com I figured it out.
Every vCenter has a Unique ID. (administration – vCenter server settings – Runtime settings) The value can be between 0 and 63. The value is generated during the installation of vCenter. I checked both vCenters and they had the same unique ID 2.
vCenter then uses the following formula for MAC address generation: 00:50:56: (80+UniqueID) :00:00 This means it is only the last four digits
I got more curious about this and tried logging directly in to an ESXi host and creating two virtual machines. They got the following MAC addresses
During my research I disovered VMware have three OUI for MAC addresses
00:50:56 (Used for vCenter)
00:0C:29 (Used when creating VM directly on an ESXi host)
00:05:69 (Maybe used on ESX?)
When going through the VMware knowledge base there was a document stating that you should use 00:50:56 for static MAC address. Use the range 00:50:56:(00-3F) for your static mac addresses