Networking - What is ARP and how does it work?

[radhika_btech]

What is ARP and how does it work? <?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" /><o:p></o:p>

[muralipn]

Hi Radhika,

ARP:
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When an Ethernet frame is sent from one host on a LAN to another, it is the 48-bit Ethernet address that determines for which interface the frame is destined.

Address resolution provides a mapping between the two different forms of addresses: 32-bit IP addresses and whatever type of address the data link uses.

Working of ARP:

1. The application, the FTP client, calls the function gethostbyname to convert the hostname into its 32-bit IP address. This function is called a resolver in the DNS (Domain Name System).
2. The FTP client asks its TCP to establish a connection with that IP address.

3. TCP sends a connection request segment to the remote host by sending an IP datagram to its IP address.

4. IP datagram is sent to a host or router on a locally attached network.

5. The sending host must convert the 32-bit IP address into a 48-bit Ethernet address.

6. ARP sends an Ethernet frame called an ARP request to every host on the network. This is called a broadcast.

7. The destination host's ARP layer receives this broadcast, recognizes that the sender is asking for its hardware address, and replies with an ARP reply.

8. The ARP reply is received and the IP datagram that forced the ARP request-reply to be exchanged can now be sent.

9. The IP datagram is sent to the destination host.

Regards,
Murali

[jithendra.b]

ARP stands for Address Resolution Protocol. The primary objective of ARP is to obtain the physical
address for the logical address.

Example : Consider that system A wants to transfer a packet to system B on the other side. In
order to transfer the packet ,it needs to be stuffed with additional details like address of the destination etc.

At first system A sends an ARP request to all the machines that are on the other side. This is called broadcasting. The ARP request frame contains the sender's logical and physical address followed by the logical address of the destination.This frame is received by all the machines at the destination. Now the machine whose logical address matches with the one present in the frame processes the frame and other ignore it.

Then the machine(the one which had processed the packet) sends a reply to the source(system A ).This reply contains the physical address of the machine.

This is how ARP works.