The following is the reason for ARP request to be sent as broadcast message:
• For a datagram to be sent to the destination node, the source node should know the IP (Internet protocol) address and MAC (Media Access Control) address of the destination node.
• Source node with IP address of destination node will send ARP query as broadcast message across the LAN to know the MAC address of the destination node.
• All nodes will receive this ARP query message. ARP module present at each node will check whether the IP address in ARP query matches with its IP address.
• If the IP address matches then the node will send the ARP response with MAC address of the node.
• Then the source node will know the MAC address of the destination node.
Hence, ARP query is sent as broadcast message by the source node to get the unknown MAC address of destination node.
The ARP response is sent within a frame with specified destination MAC address because the node that sends the ARP response knows the MAC address of source node (which has sent the ARP query).
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