When you install DNS on a Windows server that does not have a connection to the Internet, the zone for the domain is created and a root zone, also known as a dot zone, is also created. This root zone may prevent access to the Internet for DNS and for clients of the DNS. If there is a root zone, there are no other zones other than those that are listed with DNS, and you cannot configure forwarders or root hint servers.
Root domain
This is the top of the tree, representing an unnamed level; it is sometimes shown as two empty quotation marks (""), indicating a null value. When used in a DNS domain name, it is stated by a trailing period (.) to designate that the name is located at the root or highest level of the domain hierarchy. In this instance, the DNS domain name is considered to be complete and points to an exact location in the tree of names. Names stated this way are called fully qualified domain names (FQDNs).
DNS Domain Name Hierarchy: