How to allow connections to a port from specific IP addresses only?
Go to Tools & Settings > Firewall.
Note: If Firewall is not available, install it using the instructions from this KB article.
Enable Firewall Rules Management and click Modify Plesk Firewall Rules.
Click on an existing rule to edit.
Select Allow from selected sources, deny from others and add IP addresses from which connections will be allowed.
Click Apply Changes.
Alternatively, use the OS native tool to achieve that:
For Windows (Windows Firewall)
For Linux (iptables)