The data link layer is concerned with addressing at the physical machine level. Protocols at this layer are involved with communications controllers, their chips, and their buffers. Ethernet is supported at this layer by TCP/IP.
Two additional TCP/IP protocols, ARP and RARP, can be viewed as existing between the network and data link layers. ARP is the Ethernet address resolution protocol. It maps known IP addresses (32 bits long) to Ethernet addresses (48 bits long).
RARP is the IP reverse address resolution protocol. It maps known Ethernet addresses (48 bits long) to IP addresses (32 bits long), the reverse of ARP. For more information about ARP and RARP, refer to the arp(7tcp) manual page.