You perform a network scan using ICMP Echo Requests and observe that certain IP addresses...

According to CEH v13 Network Scanning and Enumeration, ICMP Echo Requests (ping) are commonly filtered by firewalls and intrusion prevention systems to reduce network reconnaissance exposure. When ICMP Echo Replies are not returned but other services remain operational, the most likely explanation is ICMP filtering rather than host unavailability or compromise.

CEH v13 explicitly states that many organizations configure firewalls to block ICMP Echo Requests while allowing other ICMP types or higher-layer protocols. This practice helps prevent attackers from easily mapping live hosts during the reconnaissance phase.

The other options are incorrect because:

Unused IPs would not necessarily have active services.

A breach would typically present additional symptoms.

Network congestion would affect multiple protocols, not just ICMP.

Thus, blocked ICMP is the correct interpretation.