In this article, I will go over the basic concept of ARP in networking and how it actually works and what it does

What is ARP?

• ARP stands for Address Resolution Protocol.

• It is a protocol in the Network Layer (Layer 3) of the OSI Model or TCP/IP Model.

• The responsibility of this protocol is to map the IP Addresses to their respective MAC Addresses in a LAN (Local Area Network).

How Does it Work?

• Systems keep an ARP look-up table where they store information about what IP Addresses are associated with which MAC Addresses.

• It is important to remember to understand what is a MAC Address and IP Address

  • A MAC Address is the unique identifier of the device/hardware. This is "burned" into the device and can not change.
  • An IP Address is the unique identifier of a device on a network.

• When trying to send a packet to an IP address, it will first check the table to see if it is already listed.

• If the value is cached, ARP won't be used and it will just easily identify the device and send the packet over.

• However, it it isn't found, the system will send a broadcast packet to the network using the ARP protocol to ask "who's IP Address is this?"

• Because this is a broadcast packet, it will be sent to a special MAC Address that causes all devices on the network to receive it.

• Since, every device/machine on the network will receive it, the device with that specific IP Address will send back an ARP packet confirming that "I am that IP Address."

• You could compare this to a roll-call. Someone will call for "Is Billy here?". Since everyone is here during this roll call, Billy will respond back with "I am here."

• After this mapping is complete, the data is ready to be passed down to Layer 2 or the Data Link Layer and transmitted.