ARP\RARP

ARP (Address Resolution Protocol)

ARP maps logical address to physical address.

ARP Packet Format

An ARP packet in encapsulated directly into data link frame (Ethernet frame).

An ARP request is a broadcast and an ARP reply is unicast.

Proxy ARP

A technique called proxy ARP is used to create a subnetting effect. A proxy ARP is an ARP that acts on behalf of a set of hosts.

Whenever a router running a proxy ARP receives an ARP request looking for the IP address of one of these hosts, the router sends as ARP reply announcing its own hardware (physical) address.

RARP (Reverse Address Resolution protocol)

RARP finds the logical address for a machine that only knows its physical address.

A diskless machine is usually booted from ROM. A RARP request is created and broadcast on the local network.

Another machine on the local network that knows all the IP addresses will respond with a RARP reply. The requesting machine must be running a RARP client program, the responding machine must be running a RARP server program.

The RARP request packets are broadcast and the RARP reply packets are unicast. A RARP packet is encapsulated directly into a data link frame.

RARP Packet Format