Reverse Address Resolution Protocol

Mapping physical address to logical address

In many situations, a host or router knows its MAC address but needs to know its logical (IP) addresses.
RARP is used to find the logical addresses for a machine that knows its physical address.
Each host or router can be assigned with one or more logical (IP) address/es. These addresses are unique and independent of the physical (hardware) address of the machine.
To create an IP datagram, a host or router is required to know its own IP address. The IP address of a machine is generally read from its configuration file stored on a disk file. A diskless machine is booted from ROM, which has a minimum booting information.
The machine gets its physical address (by reading NIC), which is unique locally. This physical address is used to get logical address by using RARP protocol.
RARP request is created and broadcasted on the local network.
So, the another machine on the local network that knows all the IP addresses responds with a RARP reply.
It is necessary that the RARP requesting machine must be running a RARP client program and the responding machine must be running a RARP server program.

Since it operates at low level, it requires direct address to the network, which makes it difficult for an application developer to built a server.
It does not fully utilize the capability of a network, like ethernet, which is enforced to send a minimum packet size, since the reply from the server contains only small piece of information i.e. 32- bit internet address.