Windows – Internal DNS for non domain users

Since the previous answers to this question were written, there have been a couple of RFCs that alter the guidance somewhat. RFC 6761 discusses special-use domain names without providing specific guidance for private networks. RFC 6762 still recommends not using unregistered TLDs, but also acknowledges that there are cases where it will be done anyway. Since the commonly used .local conflicts with Multicast DNS (the main topic of the RFC), Appendix G. Private DNS Namespaces recommends the following TLDs:

• intranet
• internal
• private
• corp
• home
• lan

IANA appears to recognize both RFCs but does not (currently) incorporate the names listed in Appendix G.

In other words: you shouldn't do it. But when you decide to do it anyway, use one of the above names.

This has been a fun topic of discussion on Server Fault. There appear to be varying "religious views" on the topic.

I agree with Microsoft's recommendation: Use a sub-domain of the company's already-registered Internet domain name.

So, if you own foo.com, use ad.foo.com or some such.

The most vile thing, as I see it, is using the registered Internet domain name, verbatim, for the Active Directory domain name. This causes you to be forced to manually copy records from the Internet DNS (like www) into the Active Directory DNS zone to allow "external" names to resolve. I've seen utterly silly things like IIS installed on every DC in an organization running a web site that does a redirect such that someone entering foo.com into their browser would be redirected to www.foo.com by these IIS installations. Utter silliness!

Using the Internet domain name gains you no advantages, but creates "make work" every time you change the IP addresses that external host names refer to. (Try using geographically load-balanced DNS for the external hosts and integrating that with such a "split DNS" situation, too! Gee-- that would be fun...)

Using such a subdomain has no effect on things like Exchange email delivery or User Principal Name (UPN) suffixes, BTW. (I often see those both cited as excuses for using the Internet domain name as the AD domain name.)

I also see the excuse "lots of big companies do it". Large companies can make boneheaded decisions as easily (if not moreso) than small companies. I don't buy that just because a large company makes a bad decision that somehow causes it to be a good decision.