This chapter discusses protocols associated with the data link layer. The protocols covered are Address Resolution Protocol (ARP), Reverse Address Resolution Protocol (RARP), link layer functions, Link Layer Discovery Protocol (LLDP), and Cisco Discovery Protocol (CDP). As mentioned earlier, the data link layer must ensure that messages are transmitted to devices on a LAN using physical hardware addresses, and they also must convert packets sent from the network layer, and convert them into frames to be sent out to the physical layer to transmit. The data link layer converts packets into frames, which adds a header that contains the physical hardware device of the source and the destination address, flow control, and a footer with the checksum data (CRC). We are going to dive deeply into this layer.