It's rare to find an organization running its own DNS that is small enough to not take advantage of subdomains and delegation. By delegation, I mean letting one group, whether logical or physical, administer a section of an organization's network. Let's take a look at an example.
Perhaps my company has two offices: one in Boston and the other in Charlotte, North Carolina. Although I have an overarching domain name, mycompany.com, I might want to delineate these two locations within my network—I can call all machines in Boston with the north.mycompany.com domain suffix and all machines in Charlotte with the south.mycompany.com domain suffix. Because the respective IT groups at each location have a better sense of which machines are going in and out of the network at their own offices than a central group of administrators at the headquarters site, the decision was made to let each office's group administer DNS within each subdomain. To make this happen, there are three steps to follow: first, the overarching domain's DNS zone needs to be told there will be a subdomain that will be administered elsewhere. Second, the overarching (in technical terms, the "root" but not the ultimate TLD-root) nameserver needs the address of the subdomain's nameserver for its records. And finally, the subdomain's nameserver needs to be installed and configured.
