NMAP (Part 1): Port Scanning Responses

This article is part of a series. Check out the full series: Part 1: Port Scanning Responses, Part 2: Basic Port Scanning Types, Part 3: Other Port Scanning Types, Part 4: Network Scanning, Part 5: Firewall Evasion Options and Part 6: NSE Scripts!

Before running any port scanning, it is important for you to first understand the 3 basic types of NMAP port scanning responses.

1. Open: What can I do for you?
2. Closed: The port is currently in use at this time. It is closed and not listening.
3. Filtered: Traffic is blocked or dropped so no response.

Closed and Filtered responses could be confusing sometimes. Just remember that closed means RST packet is received (normally from the endpoints) and filtered means nothing is received or ICMP error is received (normally from the firewalls).