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In Computer Networking , the Address Resolution Protocol ('''ARP''') is the method for finding a host's hardware address when only its IP address is known. Due to the overwhelming prevalence of IPv4 and Ethernet , ARP is primarily used to translate ethernet MAC Address es from IP Address es. It can, however, easily be used for IP over ATM or FDDI . ARP is used in four cases of two hosts communicating: # When two hosts are on the same network and one desires to send a packet to the other # When two hosts are on different networks and must use a Gateway / Router to reach the other host # When a router needs to forward a packet for one host through another router # When a router needs to forward a packet for one host to the destination host on the same network The first case is used when two hosts are on the same physical network (i.e., they can directly communicate without going through a router). The last three cases are the most used over the Internet as two computers on the internet are typically separated by more than 3 hops. Imagine computer A sends a packet to computer D and there are two routers, B & C, between them. Case 2 covers A sending to B; case 3 covers B sending to C; and case 4 covers C sending to D. ARP is defined in RFC 826. VARIANTS OF THE ARP PROTOCOL ARP was not originally designed as an IP-only protocol although today it is primarily used to map IP addresses to MAC addresses. ARP can be used to resolve MAC addresses to many different Layer 3 protocols addresses. ARP has also been adapted to resolve other kinds of Layer 2 addresses; for example, ATMARP is used to resolve ATM NSAP addresses in the Classical IP over ATM protocol. ARP MEDIATION ARP Mediation refers to the process of resolving Layer 2 addresses when different resolution protocols are used on either circuit, for e.g. ATM on one end and Ethernet on the other. INVERSE ARP The Inverse Address Resolution Protocol, a.k.a. '''Inverse ARP''' or '''InARP''', is a protocol used for obtaining Layer 3 addresses (e.g. IP Addresses ) of other stations from Layer 2 addresses (e.g. the DLCI in Frame Relay networks). It is primarily used in Frame Relay and ATM networks, where Layer 2 addresses of Virtual Circuits are sometimes obtained from Layer 2 signalling, and the corresponding Layer 3 addresses must be available before these virtual circuits can be used.. COMPARISON BETWEEN ARP AND RARP ARP translates Layer 3 addresses to Layer 2 addresses, therefore InARP can be viewed as its inverse. In addition, InARP is actually implemented as an extension to ARP. The packet formats are the same, only the operation code and the filled fields differ. RARP , like InARP, also translates Layer 2 addresses to Layer 3 addresses. However, RARP is used to obtain the Layer 3 address of the requesting station itself, while in InARP the requesting station already knows its own Layer 2 and Layer 3 addresses, and it is querying the Layer 3 address of another station. RARP has since been abandoned in favor of BOOTP then DHCP . PACKET STRUCTURE The following is the packet structure used for ARP requests and replies. Note that the packet structure shown in the table has SHA, SPA, THA, & TPA as 32-bit words but this is just for convenience — their actual lengths are determined by the hardware & protocol length fields.
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