Which ports are being used?

Netstat is a tool to get an overview of network connections and interfaces.

You can get a detailed list of ports being used of what connections with the netstat command and the following five flags. Read more about the flags in the manual $ man netstat.

$ netstat -tulpn
Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name
tcp        0      0 127.0.0.1:25            0.0.0.0:*               LISTEN      833/sendmail: MTA: 
tcp        0      0 0.0.0.0:5666            0.0.0.0:*               LISTEN      634/nrpe        
tcp        0      0 127.0.0.1:27017         0.0.0.0:*               LISTEN      338/mongod      
tcp        0      0 127.0.0.1:3306          0.0.0.0:*               LISTEN      612/mysqld      
tcp        0      0 127.0.0.1:587           0.0.0.0:*               LISTEN      833/sendmail: MTA: 
tcp        0      0 0.0.0.0:111             0.0.0.0:*               LISTEN      264/portmap     
tcp        0      0 109.202.159.44:80       0.0.0.0:*               LISTEN      1827/node       
tcp        0      0 127.0.0.1:28017         0.0.0.0:*               LISTEN      338/mongod      
tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN      790/sshd        
tcp6       0      0 :::22                   :::*                    LISTEN      790/sshd        
udp        0      0 0.0.0.0:111             0.0.0.0:*                           264/portmap     

If you only want to know what is listening on a specific port you can just grep that port. In this example port 22 (sshd):
$ netstat -tulpn | grep ':22'

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