This information may be used by the original sender as network communication error diagnostic information. Example: IPv4 header of the original packet that couldn’t delivered. If a packet couldn’t delivered to its destination, the Destination Unreachable message is returned to the sender and the Destination Unreachable message contains a portion of the datagram that could’t delivered.
ICMP Destination Unreachable messages are sent back to the sender unsolicited (not as a response to an ICMP query) from the receiver or any other device in between, like a router. The receiver may be down, wrong IPv4 address, router does’t know the way to reach the destination network etc. There may be many reasons that the packet was unable to be delivered to the receiver. As the name indicates, ICMP Destination Unreachable messages are sent back to the sender if a packet could not be delivered to the receiver.
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