To Make the DNS server act as a slave for the zone of domainname.com, please follow the following steps :-
1) Login to your Plesk control panel.
3) Click on "Websites and Domains".
4) Click on the DNS Settings.
5) Click on [Manage] in the front of that domain name only.
6) Click on "Switch DNS Service Mode".
Information: This server now acts as a slave for the DNS zone gwdemo.com.
