If you are on a LAN you can try sending a broadcast ping (ping -b 255.255.255.255 on linux) which any compliant device will respond to. Then you can get a list of MAC addresses by looking up the ARP cache (arp -a on linux).

Note that this will only get the addresses of devices on the local Ethernet segment. Not every device will respond to a broadcast ping.

If you want MAC addresses of devices beyond the current Ethernet segment (ie devices that are a router hop or more away) then you will have to get that device to volunteer the MAC address information by using a SNMP query or by installing your own special client software on each of those remote devices.