Confused about this domain's naming

We're replacing a Windows 2008 domain controller, and we're confused about the existing domain name and logins.

All existing machines are members of the domain ABCCOMPANY.COM, but when users login to the domain, they use this domain name in their credentials:  DEF\username

In Active Directory Domains and Trusts, we see one domain:  ABCCOMPANY.COM.  The Properties of that show a Domain Name of (pre-Windows 2000) of DEF .

In Active Directory Users and Computers/Domain Controllers, we see two devices:  Server1 and Server2.  Server1 is nowhere to be found; that is, it has been turned off and relegated to a closet somewhere.  It is still designated as a DC, Domain Controller.  Server2 is still quite active and is designated as the GC, Global Catalog.  We intend to de-commission Server2.

Our questions are these, but additional specific info is welcome.

1.  Why is the DEF part of the login credentials so prevalent?  Is it simply because of the pre-Windows 2000 name?

2.  When should we use DEF moving forward?

3.  We believe that the original local domain should not have a .com extension.  In moving to a new domain controller and demoting the old one, should we re-name it?  (And of course we can educate ourselves via Google, but what steps are taken to rename it?)

Thank you... as is often the case when one is confused, we're not 100% sure what to ask, so please feel free to enlighten us in this area.
K AAsked:
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MaheshArchitectCommented:
"DEF"...it is your domain NetBIOS name and your computers are getting GPOs because of this name

For login also you can use that name, but you have an option to use domain FQDN as well
Apps can query your domain on that NetBIOS name..

If you already have .com domain, its good and why you want to convert it to .local?

Its not required
U can simply use split dns setup for your internal and external name resolution
As per Microsoft practises, you should have contoso.com as external domain and corp.contoso.com as internal domain (sub domain),

I don't see need for any of two above in your setup, I seen  people struggling previously to rename .local to .com for internal AD
Since domain rename is not simple operation and impact can be fatal if not planned and executed properly because of application infrastructure
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Shaun VermaakTechnical Specialist/DeveloperCommented:
1.  Why is the DEF part of the login credentials so prevalent?  Is it simply because of the pre-Windows 2000 name?
It is your NetBIOS name

2.  When should we use DEF moving forward?
When logging on with UNC and not UPN

3.  We believe that the original local domain should not have a .com extension.  In moving to a new domain controller and demoting the old one, should we re-name it?  (And of course we can educate ourselves via Google, but what steps are taken to rename it?)
Why? There is absolutely nothing wrong with having a .com extension.
I have worked on at least 300 domains with at least a million endpoints and there is no reason to use .LOCAL extension if you configure DNS correctly
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K AAuthor Commented:
Thank you BOTH for your help so far.  So is it fair to say that the names ABCCOMPANY.COM and DEF are interchangeable?  When we join new computers to the domain we can simply use DEF as the domain name?
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QlemoBatchelor, Developer and EE Topic AdvisorCommented:
... and there is absolutely no reason to name your internal and eernal name the same. "if you configure DNS correctly" is a big issue in particular when trying to reach your external web sites.
Having said that, I would not put in effort to change the domain name is everything is working fine.

In regard to fhe NetBIOs domain name, you should be able to change that. You can always use user@ABCCOMPANY.COM for logging in if there are issues with the changed NetBIOS name.
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Michael B. SmithExchange & Active Directory ExpertCommented:
It is not supported to rename a domain if Exchange or Lync/Skype are installed in the domain. There are also serious considerations about DFS and Certificate Services and other applications.

Just say no. Don't do it. You WILL regret it.

If you don't like the current name, define a new UPN suffix, assign it to your users, and use it instead.
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Shaun VermaakTechnical Specialist/DeveloperCommented:
is a big issue in particular when trying to reach your external websites.
This is literally one DNS record if you follow correct practices of www.domain.com
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K AAuthor Commented:
Thanks to all...
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