Ports on Ubuntu — Netstat

List all listening ports
sudo netstat -plnt
Filter by a particular port
$ sudo netstat -plnt | grep ':80'
If something else is listening on the port, you can disable the program by running 
sudo service httpd stop
or change its configuration so that it no longer listens on the port. When netstat shows the port is free, enable the correct service (for example sudo service vsftpd start).

If you make any changes because the incorrect service is listening, run the netstat command again.

If netstat doesn’t show the program listening on the correct port, you need to address its configuration before you go any further.

Check status of firewall
sudo ufw status verbose

To allow/open a port 21
sudo ufw allow 21/tcp

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