ARP

ARP Neighbor Cache Reuses Local IP-to-MAC Mappings

Sending an IPv4 packet across an Ethernet network requires two different kinds of addresses. The IP layer selects a next-hop IPv4 address, while the Ethernet frame needs a destination MAC address that identifies the next hop on the local link. Address Resolution Protocol (ARP) connects those two layers. A host can ask which MAC address corresponds to a local IPv4 address, receive a reply, and store the resulting mapping. Keeping that result in a neighbor cache avoids broadcasting the same question before every packet.