This has been a fun topic of discussion on Server Fault. There appear to be varying "religious views" on le topic.
I agree with Microsoft's recommendation: Use a sub-domain of le company's already-registered Internet domain name.
So, si you own foo.com, use ad.foo.com ou certains such.
The le plus vile thing, as Je vois it, is using le registered Internet domain name, verbatim, for le Active Directory domain name. This causes you to be forced to manually copy records depuis le Internet DNS (like www) into le Active Directory DNS zone to allow "external" names to resolve. J'ai seen utterly silly things like IIS installed on chaque DC in an organization running a web site that does a redirect such that someone entering foo.com into leur browser would be redirected to www.foo.com by these IIS installations. Utter silliness!
Using le Internet domain name gains you no advantages, mais creates "make work" chaque time you changez le IP addresses that external host names refer to. (Try using geographically load-balanced DNS for le external hosts et integrating that avec such a "split DNS" situation, too! Gee-- that would be fun...)
Using such a subdomain has no effect on things like Exchange email delivery ou User Principal Name (UPN) suffixes, BTW. (I often see those les deux cited as excuses for using le Internet domain name as le AD domain name.)
I aussi see le excuse "lots of big companies do it". Large companies can make boneheaded decisions as easily (if pas moreso) than small companies. Je ne buy that juste parce que a large company makes a bad decision that somehow causes it to be a good decision.
