Which ICMP type is commonly used to advertise destination unreachable errors?

• A. Type 0 (Echo Reply)
• B. Type 8 (Echo)
• C. Type 3 (Destination Unreachable)
• D. Type 11 (Time Exceeded)

Answer: (C)

ICMP exists to report problems back to the sender. When a packet can’t reach its destination, a Destination Unreachable message is generated and sent to explain why delivery failed. This type carries codes that describe the exact reason, such as network unreachable, host unreachable, protocol unreachable, port unreachable, or fragmentation needed with the DF bit set, among others. It’s specifically about signaling that the destination can’t be reached, not about confirming a response or signaling a temporary timeout.

By contrast, Echo messages (request and reply) are used for simple reachability tests like ping, not for reporting why delivery failed. Time Exceeded messages indicate that a packet’s lifetime expired or that fragment reassembly timed out, not that the destination is unreachable.