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On Ethernet Netwok, every node need to have the ethernet address / Physical Machine Address with whom it has to communicate. It Does know it has to communicate with1.1.1.1 Host. But It simply doesnt know which machine on the network is1.1.1.1. That is identified by the MAC / Hardware Address. If that address is unknown to the Host, it sends ARP Packet (Broadcasted) asking for the MAC Address of the specified host ip1.1.1.1. the Host1.1.1.1 then replies back its mac address to that node.
Once Both Hosts have the MAC Addresses, unicast communication takes place between them.
There are two type of ARP messages
– ARP reply that is unicast to the requesting station alone
– ARP request that is broadcast to all the systems in a LAN segment
Unicast: If the MAC address is present in ARP cache (A table that contains IP address and their corresponding MAC address in the network) for corresponding IP address
Broadcast: If the MAC address is not present in its ARP cache table for corresponding IP address.
If IP address has changed to corresponding MAC address:
A Gratuitous ARP message is broadcast to all the systems of a LAN when an IP address/ MAC address of system has changed. This enables the computers in a LAN to update their ARP cache tables appropriately
Method of ARP working:
When a computer A wants to send a Packet to computer B, First computer A uses a cached ARP table to look up for Computers B IP address for any existing records of MAC address. If the Mac address is found, it sends the IP packet on the link layer to Computer’s B Mac-address. If the cache did not produce a result for Computer’s B IP address , Computer A has to send a broadcast ARP message (destination FF:FF:FF:FF:FF:FF MAC address which is accepted by all computers) requesting an answer for Computers B IP address. Computer B responds with its MAC address (and its IP). The response information is cached in Computer’s A ARP table and the message can now be sent
Very simple consider (ARP) as a way computers and network devices use to announce there presence on a network, so as a computer becomes online on a network it start sending ARP requests immediatly.
Like when you enter a room or a place and say "Hi I'm here".
Hope this was helpful.
answer is very simple.
traffic has go thru switch. and switch work on layer2 needs mac address. to get mac need arp resolution.
Once we know that source IP and destination IP is of same network, then layer2 comes in progress. But since layer2 works on mac address therefore, we need to find out that destionation IPs mac address in order to communicate with that specific host. That is why broadcast is sent to the network and only host with destionation IP will respond with its mac address and that mac address is mapped with that destionation IP and kept in cache (i.e. of ARP).
the transmetting device have to encapsulate the ip packet into an ethernet frame, for this purpose it needs to know the next hop device`s MAC address which is not known for it on the segment, hence it broadcasts arp request and the concerned node replies with a packet containing its mac address which cause address resolution.
worm regards.
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