Nmap — Network Scanner GuideStep-by-step Nmap reference: host discovery, port scans, service detection and OS fingerprinting on scanme.nmap.org. Educational use only.FAQ

What is Nmap?

Nmap (Network Mapper) is the standard open-source tool for discovering hosts and services on a network. It sends packets, reads the replies, and reports which hosts are up, which ports are open, and which software is behind them. Every security team maps its own network with it before attackers do.

Is Nmap illegal?

No. Nmap is a legitimate open-source scanner. Running it against networks you do not own or lack permission to test can be treated as unauthorized probing, so stick to your own lab and the official practice target scanme.nmap.org.

What is the difference between -sn, -sS and -sV?

-sn is ping-scan: only find live hosts, no port scan. -sS is the SYN stealth scan of the top 1,000 ports. -sV adds version detection, asking open ports which software and version they run.

What is a SYN scan?

A SYN scan starts the TCP handshake but never completes it: SYN goes out, SYN-ACK comes back, Nmap sends RST instead of ACK. The port state is learned without a full connection, so it is fast and quieter than a connect scan.

How do I scan safely without knocking things over?

Scan your own lab or scanme.nmap.org, use default timing (-T3), and avoid -T5 aggressive timing plus --min-rate floods against anything fragile. Printers, cameras and old industrial gear can crash under fast scans.

What is OS detection (-O)?

OS detection compares subtle TCP/IP behaviors — initial TTL, window size, option order — against a fingerprint database and guesses the operating system, like Linux 5.x or Windows 10. It needs at least one open and one closed port to work well.

How do I find live hosts on my WiFi?

Run nmap -sn 192.168.1.0/24 on your own network. Nmap ARPs every address and lists the ones that answer with their MAC vendor. That is how you spot an unknown device on your home WiFi.

How do I defend against Nmap scans?

Close ports you do not need, firewall the rest, run fail2ban to block repeat probers, and monitor logs. You cannot stop someone scanning you, but you can give them nothing interesting to find.

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