ARP: The Address Resolution Protocol

When hosts need to send data to each other on the same network, they use ARP to find out the hardware address to send the packet to.

The Address Resolution Protocol is used to enable devices to match MAC addresses to IP addresses for other devices on the same LAN. When devices learn a new MAC / IP address pairing, they add it to their ARP Table / ARP Cache for use in the future.

Basic ARP Process

  1. A host ( wants to send a packet to

  2. It identifies that is part of the same LAN.

  3. It looks up in its ARP table, but there isn’t an entry

  4. It sends out an ARP Request as an Ethernet broadcast to ask ‘who has’

  5. The switch broadcasts the packet to other hosts on the LAN

  6. The device which owns replies ‘I am, my MAC address is xx:xx:xx:xx:xx:xx’

  7. The host at receives the reply and adds an entry to its ARP table. It can now send the data to

The ARP Table

Devices, including hosts and routers, build up the ARP table with the information which they have learnt.

Information in the table includes:

  • IP Address

  • Physical / Hardware Address (i.e. MAC Address)

  • Interface

ARP is specified in IETF RFC 826.


An Ethernet Address Resolution Protocol

Internet Engineering Task Force

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