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Address Resolution Protocol
The Address Resolution Protocol (ARP) is the method for finding a MAC - Media Access Control - (host's link layer) address when only its Internet Layer (IP) or some other Network Layer address is known.
ARP has been implemented in many types of networks (so it is not only an IP and Ethernet protocol). It can be used to resolve many different network layer protocol addresses to interface MAC addresses. ARP is primarily used to translate IP addresses to Ethernet MAC addresses. It is also used for IP over other LAN technologies, such as Token Ring, FDDI, or IEEE 802.11, and for IP over ATM.
ARP is a Link Layer protocol because it only operates on the local area network or point-to-point link that a host is connected to.
ARP is also very often discussed in terms of the Open Systems Interconnect (OSI) networking model, because that model addresses hardware-to-software interfaces more explicitly and is preferred by some equipment manufacturers. Most often ARP is placed into the Data Link Layer (Layer 2), but it also requires the definitions of network addresses of the Network Layer.
Structure of ARP packet
There is the same packet structure used for ARP requests and replies. On Ethernet networks, these packets use an EtherType of 0x0806, and are sent to the broadcast MAC address of FF:FF:FF:FF:FF:FF.
Structure of ARP packet
Reverse Address Resolution Protocol (RARP) is also a Link layer networking protocol. It is used by a host computer to obtain its Internet Protocol (IPv4) address when it has available its MAC address, such as an Ethernet address. It has been rendered obsolete by the Bootstrap Protocol (BOOTP) and the modern Dynamic Host Configuration Protocol (DHCP), which both support a much greater feature set than RARP.
RARP requires one or more server hosts to maintain a database of mappings from Link Layer address to protocol address. MAC addresses needed to be individually configured on the servers by an administrator. RARP was limited to serving IP addresses only.
Reverse ARP differs from the Inverse Address Resolution Protocol, which is designed to locate the IP address associated with another host's MAC address. InARP is the complement of the Address Resolution Protocol used for the reverse lookup. RARP was only used for lookup of a host's own IP address.
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