<P> ARP is used for mapping a network address (e.g. an IPv4 address) to a physical address like an MAC address . ARP has been implemented with many combinations of network and data link layer technologies, like IPv4, Chaosnet, DECnet and Xerox PARC Universal Packet (PUP) using IEEE 802 standards, FDDI, X. 25, Frame Relay and Asynchronous Transfer Mode (ATM). IPv4 over IEEE 802.3 and IEEE 802.11 is the most common usage . </P> <P> In Internet Protocol Version 6 (IPv6) networks, the functionality of ARP is provided by the Neighbor Discovery Protocol (NDP). </P> <P> The Address Resolution Protocol is a request and response protocol whose messages are encapsulated by a link layer protocol . It is communicated within the boundaries of a single network, never routed across internetwork nodes . This property places ARP into the link layer of the Internet Protocol Suite . Although ARP was not developed in the OSI model, it is often described here as residing in layer 3, being encapsulated by Layer 2 protocols . </P> <P> The Address Resolution Protocol uses a simple message format containing one address resolution request or response . The size of the ARP message depends on the upper layer and lower layer address sizes, which are given by the type of networking protocol (usually IPv4) in use and the type of hardware or virtual link layer that the upper layer protocol is running on . The message header specifies these types, as well as the size of addresses of each . The message header is completed with the operation code for request (1) and reply (2). The payload of the packet consists of four addresses, the hardware and protocol address of the sender and receiver hosts . </P>

Arp operates at which layer of the osi model