The Public nameservers act as authoritative nameservers for domain names that reside in our DNS servers and you manage through the customer portal. The authoritative servers "answer" and "resolve" domain names to your IP address for the general internet population.
The resolving nameservers located on the private network act as DNS resolvers for your server. For example, sending mail from your server requires an NSlookup of the destination domain name. The private DNS servers resolve this information over the private network to keep your bandwidth usage down, reduce the load on the authoritative servers, and offer quick resolution. The private resolvers slave from our public nameservers so they are always up to date. This is a convenience service for our customers.