The MAC address may not have an associated IP address, (or its IP address might not be compatible with your hosts network/mask). To get all hosts that can respond, to answer a ping, use 255.255.255.255.
As others have suggested, use
tcpdump -env to look for ARP and MAC addresses. If you suspect there might be an IP misconfiguration, you can set your ip/netmask to some large netmask (195.88.88. netmask 184.108.40.206, which will likely take you offline.
Ping 255.255.255.255 and see if you get an address resolution. (This will cover networks from 128 - 255; set your ip to 10.10.10.10/220.127.116.11 to get the other half of the possible IPv4 addresses.