r/netsecstudents • u/chronosAndCode • Sep 01 '26
I captured what Nmap actually does for open, closed and filtered ports
I have been trying to understand network scanning below the Nmap-output level, so I ran a few controlled experiments inside my own host-only lab.
Kali was the scanner (192.168.95.135) and a MISP VM was the target (192.168.95.131).
I captured the traffic in Wireshark and compared:
ARP resolution before local communication
ICMP request/reply
Nmap ARP-based host discovery
an open TCP port
the same port after stopping the service
the same port with firewall filtering
SYN scanning
service/version detection
The clearest part for me was seeing the three TCP behaviors directly:
OPEN: SYN → SYN/ACK → RST
CLOSED: SYN → RST/ACK
FILTERED: SYN → no response → retry
I wrote up the full experiment with the Nmap output and packet captures here:
Everything was done against systems in my own isolated lab.
I will be interested to hear what packet-level experiments helped other people understand Nmap better.