Which ICMP code indicates a host is unreachable?

The correct code indicating a host is unreachable is Code 3. In Internet Control Message Protocol (ICMP), Code 3 corresponds to "Destination Unreachable," which means that the packet could not reach the destination because the host is not available or reachable on the network. This code is essential for network devices to communicate issues regarding the status of network connectivity and helps diagnose routing problems or unreachable addresses.

Understanding the context of ICMP can clarify why this code is significant. ICMP is primarily used for error messages and operational information queries in Internet Protocol networks. A Code 3 message can further specify the reason for the unreachability, such as Network Unreachable, Host Unreachable, Protocol Unreachable, Port Unreachable, and more. Each of these sub-codes provides essential insight into the connectivity issue encountered.

The other choices do not accurately represent the unreachable state. For instance, Code 1 is associated with "Host Unreachable," but it's considered a subtype under the broader classification of unreachable errors captured by Code 3. Code 0 refers to "Echo Reply," which is used in ping operations, and does not signify any unreachability. Code 9 does not exist within the standard ICMP protocol codes and is not applicable in this context