To identify active hosts without performing a full port scan, which Nmap scan option should you use?

• A. nmap -sU 192.168.1.1/24
• B. nmap -sS 192.168.1.1/24
• C. nmap -sn 192.168.1.1/24
• D. nmap -A 192.168.1.1/24

Answer: (C)

Host discovery is about figuring out which machines are up on the network without touching their ports. The best way to do that in Nmap is to use the ping scan option that performs host discovery and does not perform a port scan. It sends a mix of probes—ARP requests on local networks, ICMP echo requests, and sometimes TCP probes—to determine if a host responds, and it reports which hosts are up. Because no ports are scanned, this approach is fast and least intrusive, making it ideal for simply identifying active devices. The other options perform various port scans (UDP or TCP) or an aggressive sweep that includes service and OS detection, which goes beyond just identifying active hosts and can be slower or more disruptive. So, using the host discovery mode that disables port scanning is the correct method for identifying active hosts.