The protocol used to discover MAC addresses on a local network is ARP, the Address Resolution Protocol.
ARP maps an IPv4 address to the hardware address needed to deliver a frame on the local network. When a computer needs to send data to a nearby device but does not know that device’s MAC address, it broadcasts an ARP request asking which machine owns a particular IP address. The matching device replies with its MAC address, and the sender can then transmit the Ethernet frame.
Computers usually keep these mappings in an ARP cache, reducing the need to repeat broadcasts. ARP operates within a local subnet and is not routed across the wider Internet. DNS resolves domain names to IP addresses, while DHCP assigns network configuration such as IP addresses. RARP performed the reverse mapping and has largely been replaced by newer methods.
ARP applies to IPv4 networks. IPv6 uses Neighbor Discovery Protocol, or NDP, for comparable address-resolution functions.