1. Address Resolution Protocol
ARP

2. ARP Overview
On a local network, such as an Ethernet network, a sending station often knows the IP Address of a receiving station, but (at least initially) does not know the MAC address of that station.
Recall that in the MAC header, both the source MAC and destination MAC are required.
Thus, we need some way for a station on an Ethernet to discover the MAC address of some other station.
This is what the ARP protocol does

3. ARP Design Philosophy The design of ARP is Protocol Independent!
This means that, even though it is most commonly used to map 32-bit IPV4 addresses to 48 bit Ethernet MAC addresses, the protocol can be used for arbitrary address types.
For example, it can be used on IPV6 networks without any modification to the protocol.
Further, it could be adapted to other Layer 2 protocols, other than Ethernet, with potentially shorter or longer MAC addresses.

4. What protocol “layer” is ARP?
Uses the services of the MAC layer (layer 2)
So technically a “layer 3” protocol
Ethernet “type” field is 0x806
But, ARP messages are NOT forwarded by routers, and are “valid” only on a single physical network
By this, it is a MAC (or data-link) layer protocol

5. ARP Protocol Header, Request

6. ARP Protocol Header, Reply

7. ARP Protocol Header, Request With Ethernet 802.3 Header

8. ARP Protocol Header, Request IPV6