From Windows 2000 and Mainframe Integration, by William H. Zack, New Riders.
Windows 2000 offers several improvements over NT 4.0 that make it easier to use your servers and workstations on the Internet. Integrated DNS is one of them.
In a TCP/IP network, the Domain Name Service (DNS) locates distributed services by translating the symbolic name of an object into the specific TCP/IP address used to locate the object itself. The DNS has been integrated with the Active Directory in Windows 2000 to act as the locator service for Active Directory Objects.
The Windows 2000 DNS will be familiar to you if you've browsed the Internet, since the Windows 2000 DNS can be spliced into the big Internet DNS and used to name and offer location services to publicly available resources (i.e. Web servers, internal resources, etc.)
Because Windows 2000 integrates so closely with DNS and because DNS can then be used to provide a lookup service that can be used inside and outside a company, you should register your domain name with the Internet name registration authority. Even if you have no intention of providing access from the Internet, register your name! That leaves you the option of adding that capability later. Also, you will avoid unscrupulous individuals who register other company's domain names and then sell them back to the company at a hugely inflated rate.