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ARP PROTOCOL

ARP is a protocol used by the Internet Protocol (IP) to map IP network address (IPv4 address) into link layer address. The protocol operates at the network layer as a part of the interface between the OSI network and OSI link layer. Reverse ARP is also available and is represented as RARP, is used to discover its IP address by a host. In this case, the host broadcasts its physical address and an RARP server replies with the host's IP address. ARP is one which lies between layer 2 and 3 of the OSI model, though it is not included in this model.

(i) Mechanism

If a packet is intended for a host machine on a specific LAN reaches a gateway. The ARP program finds a physical host or MAC address that matches the IP address as per the gateway request and looks forward to ARP cache. If it finds the address, the packet is converted to the right packet length and format, then sent to the machine. If an entry is not found for the IP address, ARP extends a request packet to all the machines on the LAN. If any of the machines has that IP address, it recognises the IP address and returns the reply. ARP updates the ARP cache for further reference and sends the packet to the MAC address.
There is separate ARP Request for Comments (RFC) for Ethernet, ATM, HIPPI, Fiber Distributed-Data Interface and other protocols since each type of LAN has different protocol details.

(ii) Proxy ARP

It is a technique in which one host, a router answers ARP request envisioned for another machine. Proxy ARP is enabled, if the switch accepts an ARP request for which it has a route to the target IP address, the switch replies by transferring a proxy ARP packet containing its MAC address. The host that sent the ARP request will send its packets to the switch, which forwards them to the intended host.
There are two modes of proxy ARP as follows:
1. Restricted
2. Unrestricted

(iii) ARP Cache

The ARP Cache is a group of ARP entries that are shaped when a hostname is resolved to an IP address and then an IP address is resolved to an MAC address. The PC will store the mapped address in the ARP cache, and it will be available until the ARP cache entry timeout expires. Sometimes a bad entry of ARP can cause issues with Internet connections and Web page loading. In this situation, the resolution is to clear the ARP cache.
To View ARP Cache:
1. Open Command Prompt.
2. At the command prompt, type arp –a.

HOW TO WORK 

When an incoming packet destined for a host machine on a particular local area network arrives at a gateway, the gateway asks the ARP program to find a physical host or MAC address that matches the IP address. The ARP program looks in the ARP cache and, if it finds the address, provides it so that the packet can be converted to the right packet length and format and sent to the machine. If no entry is found for the IP address, ARP broadcasts a request packet in a special format to all the machines on the LAN to see if one machine knows that it has that IP address associated with it. A machine that recognizes the IP address as its own returns a reply so indicating. ARP updates the ARP cache for future reference and then sends the packet to the MAC address that replied.


Since protocol details differ for each type of local area network, there are separate ARP Requests for Comments (RFC) for Ethernet, ATM, Fiber Distributed-Data Interface, HIPPI, and other protocols.There is a Reverse ARP (RARP) for host machines that don't know their IP address. RARP enables them to request their IP address from the gateway's ARP cache.

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