ARP & RARP

ARP
In computer networking, the Address Resolution Protocol (ARP) is the method for finding a host's link layer (hardware) address when only its Internet Layer (IP) or some other Network Layer address is known.
ARP has been implemented in many types of networks; it is not an IP-only or Ethernet-only protocol. It can be used to resolve many different network layer protocol addresses to interface hardware addresses, although, due to the overwhelming prevalence of IPv4 and Ethernet, ARP is primarily used to translate IP addresses to Ethernet MAC addresses.

RARP
Reverse Address Resolution Protocol (RARP) is a Link layer networking protocol used by a host computer to obtain its Internet Protocol (IPv4) address when it has available its link-layer address, such as an Ethernet address.
RARP requires one or more server hosts to maintain a database of mappings from Link Layer address to protocol address. Media Access Control (MAC) addresses needed to be individually configured on the servers by an administrator. RARP was limited to serving IP addresses only.