What is ARP and RARP?

Answer Posted / suresh

TCP/IP uses the Address Resolution Protocol (ARP) and the
Reverse Address Resolution Protocol (RARP) to initialize
the use of Internet addressing on an Ethernet or other
network that uses its own media access control (MAC). ARP
allows a host to communicate with other hosts when only the
Internet address of its neighbors is known. Before using
IP, the host sends a broadcast ARP request containing the
Internet address of the desired destination system.

The ARP/RARP header structure is shown in the illustration
below.

16 32 bits
Hardware Type Protocol Type
HLen (8) Plen (8) Operation
Sender Hardware Address
Sender Protocol Address
Target Hardware Address
Target Protocol Address
ARP/RARP header structure

Hardware type
Specifies a hardware interface type for which the sender
requires a response.

Protocol type
Specifies the type of high-level protocol address the
sender has supplied.

HLen
Hardware address length.

PLen
Protocol address length.

Operation
The values are as follows:

1 ARP request.
2 ARP response.
3 RARP request.
4 RARP response.
5 Dynamic RARP request.
6 Dynamic RARP reply.
7 Dynamic RARP error.
8 InARP request.
9 InARP reply.

Sender hardware address
HLen bytes in length.

Sender protocol address
PLen bytes in length.

Target hardware address
HLen bytes in length.

Target protocol address
PLen bytes in length.

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