Port Scanning Techniques

Open ports reveal the services a host is running, which is your map of the attack surface. Port scanning determines which TCP and UDP ports are open, closed, or filtered.

Nmap offers several scan techniques, each with trade-offs in speed, stealth, and the privileges required.

Nmap reports each port in one of these states:

• open: a service is actively accepting connections.
• closed: reachable but no service listening.
• filtered: a firewall is blocking probes, so Nmap cannot tell.
• open|filtered and closed|filtered: ambiguous results.