What is the maximum length of a Windows domain + username? That is, what is the legal limit for a domain/username in Windows?
Read this
for windows 2000: http://technet.microsoft.com/it-it/library/bb726984(en-us).aspx
for windows 2003: http://technet.microsoft.com/en-us/library/cc783323.aspx
The maximum lengths are hash defined in lmcons.h. DNLEN is the maximum domain length and UNLEN is the maximum user name length.
In the version of this file I have on my XP machine (installed as part of Visual Studio 8), DNLEN = 15 and UNLEN = 256.
There is a constant UNLEN which is defined in Lmcons.h that defines the maximum length of a username
compmgmt.msc allows usernames for local account at max with 20 characters. dsa.msc allows domain user names to be 256.
来源：https://stackoverflow.com/questions/704891/windows-username-maximum-length