A port is open when there’s a process running on the machine which is listening on that port for incoming requests. For example, default web server open port is HTTP (80) and ssh is SSH (20).

Using netstat we can list open ports and active processes that listen them:

Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name    
tcp        0      0 127.0.0.1:5941          0.0.0.0:*               LISTEN      565/teamviewerd     
tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN      466/sshd            
tcp        0      0 127.0.0.1:631           0.0.0.0:*               LISTEN      9208/cupsd          
tcp        0      0 127.0.0.1:25            0.0.0.0:*               LISTEN      821/exim4           
tcp6       0      0 :::22                   :::*                    LISTEN      466/sshd            
tcp6       0      0 ::1:631                 :::*                    LISTEN      9208/cupsd          
tcp6       0      0 ::1:25                  :::*                    LISTEN      821/exim4
...
  • -n Include the numeric ports and addresses.
  • -l Include only listening sockets.
  • -p Add the process ID (PID) of the listening process/program.

Happy coding.

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