Address Resolution Protocol ARP

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

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.

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

ARP Protocol Header, Request

ARP Protocol Header, Reply

ARP Protocol Header, Request With Ethernet 802.3 Header

ARP Protocol Header, Request IPV6