The heated discussion that surrounds naming an internal Active Directory domain the same or differently than the external public DNS name lives on.
In my opinion, in these days of increasingly mobile, unmanaged device access, and with the "just works" mantra that I follow - using the same internal and external namespace is the preferred option. (i.e corporate.com for both your AD and public DNS).
By doing so, users get to use and remember a single logon and email address, and there's way less fuss when setting up user accounts. Any of the supposed security disadvantages are simple to overcome with split DNS servers, and the exceptional capabilities of today's application firewalls.
I recently found an interesting old thread on this topic, that is replicated below. It's a good read on the topic.
There are different answers to this classic question and while these answers ultimately depend upon company preference, much of the direction will be based upon administrator experience. The three basic scenarios outlined below are the most commonly given answers to the question, sometimes altogether and sometimes not. Some company networks use a combination of these scenarios. When explaining it to a relative beginner asking the question, many responses omit explanatory detail about all the scenarios, for fear of causing more confusion.
All three approaches will have to take both security and the end-user experience into perspective. This perspective is colored by company size, budget, and experience of personnel running Active Directory and the network infrastructure (mostly with respect to DNS and VPN). No one approach should be considered the best solution under all circumstances. For any host name that you wish to have access from both your internal network and from the external Internet you need scenario 1, although it is the most DNS-intensive over time. If you do not select this option and go with scenario 2 or 3 only, consideration will have to be given to the fact that company end-users will need to be trained on using different names under different circumstances (based on where they are (at work, on the road or at home).
Scenario 1
Choosing the same name internal/external (spilt-zone, or split-brain, whatever you want to call it) has the most administrative overhead. Why chosen? Either because a misunderstanding of the pros/cons, political, or for ease of use.
Pros:
1. Their email address is their logon name. Easier to remember.
2. Security. Each DNS zone is authoritative for the zone of that name so therefore the external DNS zone and internal AD/DNS zone will NOT replicate with each other thereby prevent internal company records to be visible to the outside Internet.
3. Short namespace. Users don't have to type in (or see) a long domain name when accessing company resources either internally or externally. Names are "pretty".
Cons:
1. Any changes made to the public DNS zone (such as the addition or removal of an important IP host such as a web server, mail server, or VPN server) must added manually to the internal AD/DNS zone if internal users will be accessing these hosts from inside the network perimeter (a common circumstance).
2. VPN resolution is problematic at best. Company users accessing the network from the Internet will easily be able to reach IP hosts in the public DNS zone but will not easily reach internal company resources inside the network perimeter without special (and manual) workarounds such as maintaining hosts files on their machines (which must be manually updated as well everytime there is a change to an important IP host in the public zone), entering internal host data on the public zone (such as for printers, SRV records for DCs, member server hosts, etc), which exposes what internal hosts exist, or they must use special VPN software (usually expensive), such as Cisco, Netscreen, etc, which is more secure and reliable anyway.
Scenario 2
Choosing a child name or delegated sub domain name of the public zone. This is one recommendation. Name such as 'ad.domain.com', or 'corp.microsoft.com'. The AD DNS domain name namespace starts at corp.domain.com and has nothing to do with the domain.com zone.
Pros:
1. Mimimal administrative overhead.
2. Forwarding will work.
3. The NetBIOS name will be 'AD' or 'CORP', depending on what you chose and what the users will see in the three-line legacy security logon box.
4. Like Scenario 1, this method also isolates the internal company network but note this at the same time is also a disadvantage (see below).
5. Better than Scenario 1, internal company (Active Directory) clients can resolve external resources in the public DNS zone easily, once proper DNS name resolution mechanism such as forwarding, secondary zones, or delegation zones are set up.
6. Better than Scenario 1, DNS records for the public DNS zone do not need to be manually duplicated into the internal AD/DNS zone.
7. Better than Scenario 1, VPN clients accessing the internal company network from the Internet can easily navigate into the internal subdomain. It is very reliable as long as the VPN stays connected.
Cons:
1. Confusion on users if they decide on using their UPN.
2. While there is security in an isolated subdomain, there is potential for exposure to outside attack. The potential for exposure of internal company resources to the outside world, lies mainly in the fact that because when the public zone DNS servers receives a query for subdomain.externaldnsname.com, they will return the addresses of the internal DNS servers which will then provide answers to that query.
3. Longer DNS namespace. This may not look appealing (or "pretty") to the end-users.
4. Security. We are assuming that we can only access the internal servers thru a VPN and assuming they are in a private subnet, they won;'t be accessible. Also assuming to secure the VPN with an L2TP/IPSec solution and not just a quick PPTP connection. If this is all so, we can assume it is secure and not accessible from the outside world.
The scenario is the recommendation from the Windows Server 2003 Deployment Guide. It states to the external registered name and take a sub zone from that as the DNS name for the Forest Root Domain:
http://www.microsoft.com/resources/documentation/windowsserv/2003/all/deployguide/en-us/default.asp
Scenario 3
Choosing a different TLD: Choosing a different TLD, such as domain.local, domain.corp, domain.net, etc. This option is usually best for either beginners or the expert, because it's the easiest to implement primarily because it prevents name space conflicts from the very beginning with the public domain and requires no further action on your part with respect to that.
But this option does makes VPN resolution difficult (like option 1) and Exchange headers when examined closely will show the company internal AD name which looks unprofessional. You can use any extension you want here such as .ad, .int, .lan, etc...
Pros:
1. Easy to implement with minimal administrative overhead. Requires minimal action on administrators.
2. Prevents name space conflicts with external domain name.
3. Forwarding works.
Cons:
1. Domain name may look unprofessional.
2. VPN resolution difficult (like option 1). That can be a sticky issue and depending on the VPN client will dictate whether it will work or not. I know one of the other MVPs (Dean Wells) created a little script to populate a user's laptop or home PC's hosts file with the necessary resources and would remove them once the VPN is dissolved.
3. Exchange HELO name must be altered (to accomodate anti-spam, SPF, and RBL software), via MetaEdit, Metabase Explorer and thru the SMTP VS properties.