The new MAC address for this network adapter. Leave blank to keep existing or default value.

A given address is set as a manual (user-supplied) address. vCenter only accepts those from the right ranges: in the 00:50:56 block that is 00:50:56:00:00:00 - 00:50:56:3F:FF:FF (00:50:56:80:00:00 - 00:50:56:BF:FF:FF is what vCenter itself generates), and 00:0C:29 is what standalone ESXi hosts generate. The form warns about addresses that are likely to be refused.

If vCenter does refuse the reconfiguration, it is retried once with the address declared as one that vCenter "assigned", rather than a manual one; if that fails too, both reasons are reported.