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://forum.networklessons.com/t/how-does-a-switch-learn-mac-addresses/865/102?u=lagapides
https://www.reddit.com/r/techsupport/comments/emulwx/what_does_an_all_zeros_mac_address_mean/
https://en.wikipedia.org/wiki/Address_Resolution_Protocol#Packet_structure