Internet Access Considerations
For your organization to be visible on the Internet, you must have an external namespace, a public namespace that anyone on the Internet can access. An Internet name authority must assign to you a DNS domain and make sure that a parent DNS zone includes a delegation to the DNS server that is authoritative for the DNS domain. However, to help prevent malicious access of your network, you can use an internal namespace, a private namespace that only users within your organization can see, preventing unauthorized people from learning the names and IP addresses of the computers on your network.
If you plan to have both an internal and external namespace, you must configure your DNS servers to enable internal clients to resolve names in both namespaces. How you plan your namespace depends on the type of clients that you have.