Sometimes it is necessary to change MAC address of a virtual machine and in most cases there is no any issue except MAC-addresses which belong to reserved VmWare MACs. In this situation ESXi shows such error:
Unable to manually change MAC address for virtual machine Failed to start the virtual machine. Module DevicePowerOn power on failed. Could not set up "macAddress" for ethernet2. Invalid MAC address specified. 00:0c:29:bd:e6:18 is not an allowed static Ethernet address. It conflicts with VMware reserved MACs.
To fix the problem you can use the algorithm described below (all information about it you can find in this article).
Add or change MAC address information to the .vmx file on the ESX/ESXi host To add or change the MAC address of a virtual machine: Shut down the virtual machine. Right-click the virtual machine and click Remove from Inventory. Login as the root user to a console session on the ESX/ESXi host. For more information, see Tech Support Mode for Emergency Support (1003677). Go to the virtual machine home directory: # cd /vmfs/volumes/DatastoreName/VMFolderName Make a backup of the existing virtual machine's .vmx configuration file with the command: # cp vmname.vmx vmname.vmx.old Open the .vmx configuration file in an editor such as vi or nano and add or change the following entries: ethernetN.checkMACAddress = “false” ethernetN.addressType = “static” ethernetN.Address = “XX:XX:XX:XX:XX:XX″ Where XX:XX:XX:XX:XX:XX is the new desired MAC address for the virtual machine. Note: For more information, see Editing files on an ESX host using vi or nano (1020302). Register the virtual machine back to the inventory. For more information, see Registering or adding a virtual machine to the inventory (1006160). Start the virtual machine.
Some old versions of ESXi distributions have a bug which doesn’t allow to change MAC address (expression ethernetx.checkMACAddress is not working). For ESXi 5.1 you have to apply this patch: ESXi510-201307401-BG. You can download it from this page.