ARP stands for Address Resolution Protocol. This is IP's way of requesting a MAC address (or hardware address) from a host located at a certain IP address. In order for a host on a network to send a unicast packet across the network, it must know both the source and destination MAC addresses. ARP is used to obtain a MAC address similar to the way DNS is used to obtain an IP address from a domain name (www.anydomain.com).
Gratuitous in this case means a request/reply that is not normally needed according to the ARP specification (RFC 826) but could be used in some cases. A gratuitous ARP request is an AddressResolutionProtocol request packet where the source and destination IP are both set to the IP of the machine issuing the packet and the destination MAC is the broadcast address ff:ff:ff:ff:ff:ff. Ordinarily, no reply packet will occur. A gratuitous ARP reply is a reply to which no request has been made.
The purpose of an arp request is to map an IP address to a MAC address for the link layer to transmit a packet. For example, ARP will get a request to map 1.2.3.4 to a MAC address 12:fd:23:gf .Then ARP will cache the address mapping that next time an arp request doesnt need to happen
An ARP request is a broadcast Ethernet packet, that is, a packet sent onto the local physical network that all attached devices will receive. A device sends an ARP request to make the query "what is the MAC address (a.k.a. hardware address, link layer address, etc.) of this IP address (a.k.a Layer 3 address, logical network address, etc.)?" An ARP reply is an unicast Ethernet packet, sent from the device that currently owns the specified IP address, back to the device which sent the ARP request. That is, no other device will receive this packet. The ARP reply answers the requester's question, saying "IP address x.x.x.x is associated with MAC address xx:xx:xx:xx:xx:xx".
ARP is a protocol that maps MAC addresses to IP addresses for packet delivery and for finding out which device in a LAN has a given IP address.
Dynamic ARP table entries are created whne a client makes an ARP request that cannot be satisfied by data already in the ARP table.
an arp (address resolution protocol) comes into picture when we have the ip address of our destination but we dont know the mac address of our destination.Normally when we open a web site this is the process which takes place.IP packet transmission comes after arp when the destination mac address is obtained after arp request.so an ip packet contains both source & destination IP & mac addr.
Dynamic ARP table entries
An ARP request may be initiated anytime a client needs to know the IP address of a machine from its MAC address.
proxy ARP
A host on a network needs to broadcast an ARP request to advertise its Mac address. The networking world is very chatty in nature and when a new network host is available it immediately broadcasts its Mac address as with a ARP message. Also when a particular network host needs to send a data packet to another network host available in the same LAN whose Mac address is unknown, the first network host sends out a ARP message requesting for the destination network hosts MAC address.
stop cheating
ARP is not used for internetwork communications, it will send ARP to find gateway to be able to communicate with another network.