What is ARP?

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

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.