I run multiple websites under one ISPConfig3 installation (Version: 3.1.5) with one IP address (i.e. the ISP control panel and all the websites are hosted in one server with one IP address). The domain names of these websites are, for example, websiteone.info, websitetwo.com and websitethree.co.nz. The hostname of this server is hostname.websiteone.info. Is it able to create multiple nameservers (could be used as name servers in the domain names registrar system to create DNS zone file), e.g. ns1.websiteone.info and ns2.websiteone.info in this server with only one IP address? Is it possible to create nameservers ns1.websitetwo.com, ns2.websitetwo.com, ns1.websitethree.co.nz and ns2.websitethree.co.nz in the same server also? I think it should be fine to do all these in the same server with one IP address which hosting the ISPConfig3 and multiple websites. So, in the domain names registrar system, if assigning the name servers to the domain names (i.e. websiteone.info, websitetwo.com or websitethree.co.nz) with the corresponding nameservers created in this ISPConfig3 server, is it going to work? There is only one IP address (i.e. the IP address of this ISPConfig3 server) for these created nameservers to point to in the domain names registrar system, is this ok?
It can work technically, the only issue would be if the registrar allows you to set all of the nameservers for a domain to names with the same address, I have seen some that allow it and others that don't over the years.
There is tutorial: https://www.howtoforge.com/tutorial/setting-up-your-own-name-service-with-ispconfig/ In general the registrar requires two name servers. You could use you ISPConfig host as one, and get the cheapest virtual server from somewhere and use that as second. If you can install ISPConfig on that cheap host, you can set it up as mirror of your first name server.