DNS on Windows Server 2003, 3rd Edition

Removing Parent Aliases

Although parent-level aliases are useful for minimizing the impact of moving your hosts, they’re also a crutch of sorts. Like a crutch, they’ll restrict your freedom. They’ll clutter up your parent namespace when one of your motivations for implementing a subdomain may have been making the parent zone smaller. And they’ll prevent you from using the names of hosts in the subdomain as names for hosts in the parent zone.

After a grace period—which should be well advertised to users—you should remove all the aliases, with the possible exception of aliases for extremely well-known Internet hosts. During the grace period, users can adjust to the new domain names and modify scripts and the like. But don’t get suckered into leaving all those aliases in the parent zone; they defeat part of the purpose of DNS because they prevent you and your subdomain administrator from naming hosts autonomously.

You might want to leave CNAME records for well-known Internet hosts or central network resources intact because of the potential impact of a loss of connectivity. On the other hand, rather than moving the well-known host or central resource into a subdomain at all, it might be better to leave it at the parent zone level.