Is it legal to scan ports on IP addresses I don't own?

Port scanning without authorization is illegal in many jurisdictions and violates computer fraud laws. Only scan systems you own, have explicit permission to test, or are authorized to audit. Always obtain written consent before conducting security assessments on systems belonging to others or third-party networks.

What's the difference between open, closed, and filtered ports?

Open ports actively accept incoming connections and indicate a service is listening. Closed ports reject connections, meaning no service is running but the port is accessible. Filtered ports don't respond clearly because a firewall, router, or host-based filter is blocking the probe, making it impossible to determine the actual port state.

How long does a port scan typically take?

Scan duration depends on several factors including the number of ports scanned, timeout settings, network speed, and target responsiveness. A scan of common ports (1-1024) typically completes in seconds to minutes, while a full range scan (1-65535) may take 10-30 minutes or longer depending on configuration.

Can port scanning be detected by firewalls or intrusion detection systems?

Yes, port scanning activities are commonly detected and logged by firewalls and intrusion detection systems (IDS). Aggressive scans with short timeouts are more easily detected. Slower, stealthier scans may evade detection but take longer. Any significant scanning activity should be documented and authorized to avoid triggering security alerts.

What should I do if I find unexpected open ports on my server?

If you discover unexpected open ports, immediately investigate which services are running on those ports using tools like netstat or ss. Determine if they're necessary for your operations. If not, stop the service, close the port, and update your firewall rules. Regularly monitor your systems to prevent unauthorized services from running.

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