Which network layer protocol is used by routers to send a 'Destination Network Unreachable'...
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Which network layer protocol is used by routers to send a 'Destination Network Unreachable' error message back to the originating host?
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Answer: A. ICMP
Internet Control Message Protocol (ICMP) Type 3 messages report destination unreachable errors back to the sending IP host.
Step-by-step Derivation:
Step 1: When a router cannot route an IP datagram because the route is down or non-existent, it drops the packet.
Step 2: It generates an ICMP Destination Unreachable message (Type 3) to notify the sender.