NetBIOS domain name field has a 15-character length limitation in Windows 2000

When you attempt to join a Windows 2000 domain, you are unable to specify a NetBIOS domain name that is longer than 15 characters.

This issue occurs because the NetBIOS domain name has a 15-character limitation.

To work around this issue, you can specify your NetBIOS domain name from left to right using a unique name (up to 15 characters) in your enterprise that matches a portion of your DNS domain name. For example, if your DNS Fully Qualified Domain Name (FQDN) is support.us.microsoft.com, and your enterprise also has a DNS FQDN of support.uk.microsoft.com, you can create a NetBIOS domain name called support-us.

Note We do not recommend using dotted netbios names. Instead, use a hyphen when a dot is needed.

This behavior is by design.

Traditionally, DNS has been the method used to resolve Internet Domain Names. In Windows 2000, DNS is used to perform Active Directory name resolution as well. The NetBIOS domain name was introduced in Microsoft LAN Manager, and is used in Windows 2000 mixed-mode environments. The 15 character length limitation is a NetBIOS limitation, not a DNS limitation.