MAC address of all zeros
A MAC address of all zeros is a special case MAC address. It is used to refer to the localhost and corresponds with the 127.0.0.1 localhost IPv4 address.
A MAC address of all zeros looks like the following:
00:00:00:00:00:00
It is also sued in ARP requests in the target hardware address field whenever this is unknown, but in such cases, the field is ignored.
Links
https://en.wikipedia.org/wiki/Address_Resolution_Protocol#Packet_structure