ARP - Address resolution protocol

On the same physical network, a host can find out the physical address of another host given only the target host’s IP address.

How ARP works?

Consider Host A want to send data to Host B.

Host A broadcasts a packet asking host with IP address IPB to respond with itsphysical address. When host B receives the packet, it will respond to Host A with its physical address. Later Host A uses Host B’s physical address to send data directly.

Systems that use ARP will maintain a cache where it will store all IP address and corresponding physical address when it receives ARP replies.Cache is a component that stores data so that future requests for that data can be served faster.

ARP PACKET

0               7  815  16                 23  24                                            31

HARDWARE TYPE		PROTOCOL TYPE
Hardware address Length	Protocol address Length	Operation Code
Sender Hardware Address (0-3 Octets)
Sender Hardware Address(4-5Octets)		Sender IP Address(0-1Octets)
Sender IP Address(2-3Octets)		Target Hardware address(0-1Octets)
Target Hardware address(2-5Octets)
Target IP address(0-3Octets)

HARDWARE TYPE                        HARDWARE Interface type

0                                                           Ethernet

PROTOCOL TYPE                         Networklevel protocol address

      0x0800                                                 IpV4

Operation Code                               Request-Response

     1                                                           ARP request

     2                                                           ARP response

When ARP request is made by Host A, the Target Hardware address fields will be empty. The missing fields will be filled by Host B and it sends ARP reply.