from doyle, vol 1 routing tcpip
A device needing to discover the data-link identifier of another device will create an ARP Request packet. This request will contain the IPv4 address of the device in question (the target) and the source IPv4 address and data-link identifier (MAC address) of the device making the request (the
sender). The ARP Request packet is then encapsulated in a frame with the sender’s MAC
address as the source and a broadcast address for the destination.