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Recently I setup a test environment for my self-test. On a win2008 I add ADDS role. I type in "domain.com" as the domain name. Everything is fine and installation completed without problem. However I don't get why everywhere the ".com" is missed. On the logon screen it shows "domain/user". When type in "whoami" in cmd it shows "domain/user".

I think it is by purpose but what is the purpose? Also what if I use domain.net or domain.co.jp as AD domain name?

Thanks.

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The name you're seeing is the NetBIOS (think legacy) name). When you configured the domain, you would have been asked for the domain name (domain.com) and later, the NetBIOS domain name (which you probably left as domain).

The NetBIOS name dates from before Active Directory, and lives to support legacy apps. You can log in using any reasonable form of your logon name:

  • User@domain.com
  • domain\user
  • domain.com\User

The same would apply if you named the original DNS domain "example.co.jp", and the corresponding NetBIOS name "example" - you would then be "example\user". You don't have to have matching names either - domain.com.au could have the NetBIOS name "AUS".

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What if I have both domain.com and domain.net in the same forest? Will it lead to conflict? –  Mark Mar 28 at 2:46
    
That's OK - but they won't be able to have the same NetBIOS (short) name. When you go through the AD Domain Controller Promotion wizard, you'll be told that the domain "DOMAIN" is already in use, and you'll have to choose an alternative NetBIOS domain name for the second domain tree. As I mentioned, the names don't have to match - so you might end up with something like domain.com being "DOMAIN" and domain.net being "NETDOMAIN" or "DOMAIN2". Anything that satisfies the NetBIOS name rules will be fine (15 or fewer letters, numbers, and from memory, one or two of the punctuation characters). –  DavidRa Mar 28 at 9:37
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What you're seeing is the NETBIOS domain name. What you typed in was the DNS domain name. The NETBIOS domain name primarily still exists to provide support for legacy systems, but either are valid.

Active Directory domain names are usually the full Domain Name System (DNS) name of the domain. However, for backward compatibility, each domain also has a pre-Windows 2000 name for use by computers running pre-Windows 2000 operating systems. The pre-Windows 2000 domain name can be used to log on to a Windows Server 2003 domain from computers running pre-Windows 2000 operating systems using the DomainName\UserName format. This same format can also be used to log on to a Windows Server 2003 domain from computers running Windows 2000, Windows XP, or servers running Windows Server 2003. Users can also log on to computers running Windows 2000, Windows XP, or servers running Windows Server 2003 using the user principal name (UPN) associated with their user account.

Microsoft has a support KB article about naming conventions, and valid/reserved names in Active Directory, here.

If you want to log in using the DNS domain name, you do so with the below format:

username@DNSdomainname

Where as logging in with the NETBIOS name is done in the following format:

NETBIOSdomainname\username

Regarding using "domain.net" or "domain.co.jp," either are valid DNS domain names for active directory, though, since you didn't name your domain with them, they wouldn't work for you. If you wanted to try to use those anyway, you could potentially do so, by setting them up as alternate UPN suffixes for your users.

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