Physical address of a device.
IP address locates the device, MAC identifies it.
Devices need it to communicate on the local network.
They get it with ARP.
Used to resolve IP addresses to MAC addresses.
The broadcast does not go beyond the router.
Device A - Who is 10.0.0.4? I need the MAC address.
Device B - I am 10.0.0.4. I will send you the MAC address.
This is all cached to avoid sending the signal again.
arp -a
IP addresses can be matched to MAC manually.
arp -s 10.0.0.4 90-02-7b-c2-c0-67