When a host receive a packets it will try to match it 1st with the mac addresses and if it doesnt know the mac address it will drop it unless the mac address is FF:FF:FF:FF:FF:FF
Because they will reconize it as a broadcast .
FF:FF:FF:FF:FF:FF = 255.255.255.255
SUBNET BROADCAST OR DIRECT BROADCAST
A Subnet or Direct broadcast is targetted not to all hosts on a network, but to all hosts on a subnet.the purpose of this special broadcast is to send a message to all the hosts in a particular subnet.