SYN Scan (default)
nmap -sS 10.10.10.5
Sends SYN, waits for SYN-ACK. Never completes the handshake. Fast, stealthy(ish), requires root.
UDP Scan
nmap -sU 10.10.10.5
Slow. ICMP port-unreachable = closed. No response = open|filtered.
Version Detection
nmap -sV 10.10.10.5
Sends probes to open ports, matches against service fingerprint DB.
OS Fingerprinting
nmap -O 10.10.10.5
Needs at least one open and one closed port to work reliably.