Active Directory domain name length

I have a customer that has a external domain name which is 23 characters long that they want to use for a new office 365 tenant so as an example domain looks like this . I need to build them a new internal active directory domain and was going to use but was wondering if a active directory domain name with 23 characters not including the int. or .com is to long ?
JamesSolutions ArchitectAsked:
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Lee W, MVPTechnology and Business Process AdvisorCommented:
It should be fine.  Just don't go beyond 15 in the netbios name (I STRONGLY recommend the netbios name be something VERY VERY SHORT - for example, if the company is "Twenty Three Characters Inc" then the internal NETBIOS name should be "TTC"

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JamesSolutions ArchitectAuthor Commented:
Great, thanks for that. The company name is 3 words so I can make the netbios name 3 characters to keep it short.

I was thinking of shortening the internal AD name but i thought it was best practice to have the internal AD domain a sub-domain of the external.
Sajid Shaik MSystem AdminCommented:
onpremise domain and azure ad are different things... you can create a domain and sync users ... it sync only UPN and Password which authenticates at office 365 by azure ad..

for detailed guidiance please check the following

all the best
Shaun VermaakTechnical SpecialistCommented:
You can probably then just add an shorter UPN, change it on users and sync
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