Thank you for your reply.
Well, at the moment I set up just one name server in ispconfig:
I also created a site www.mydomain.com
Also I changed the nameservers where I registered mydomain.com to:
ns1: ns1.mydomain.com (the registrar allows just one ns)
There I also created a host:
ns1.mydomain.com that points to my ip.
HOWEVER: when I try to ping ns1.mydomain.com the following happens:
the correct ip is displayed, but then no packets are exchaned - there's 100% losses.
Where is the mistake?