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.
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
Dynamic ARP table entries are created whne a client makes an ARP request that cannot be satisfied by data already in the ARP table.
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.
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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.
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ARP is not used for internetwork communications, it will send ARP to find gateway to be able to communicate with another network.
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".
A broadcast to all the nodes on its segment
A broadcast to all the nodes on its segment
ARP is used to find a MAC (layer-2) address, if you know the IP (layer-3) address.First, a device will search its ARP cache, to see whether it already happens to have the required address. If it doesn't find the address, it will send an ARP request as a broadcast, which basically asks "Who has such-and-such an IP address?" The machine that has the requested IP address will send an ARP reply.