ARP

Your computer has determined: 192.168.1.50 is on my local network. But Ethernet doesn't deliver using IP addresses. It needs a MAC address. MAC stands for Media Access Control. An Ethernet MAC address is 48 bits (6 bytes), usually written as six pairs of hexadecimal digits (0–9 and A–F), such as AA:BB:CC:DD:EE:FF. So: Which MAC address owns 192.168.1.50?

Your computer broadcasts: Who has 192.168.1.50?

The owner replies:
192.168.1.50 → AA:BB:CC:DD:EE:FF

Your computer can now address the Ethernet frame correctly.