User Logon Names

In native mode, do users have to use their entire AD logon name in order to access the network? In other words, if User1 is in the OU’s East and in a nested group within East called MFG, would they have to login using “User1@domain.com\East\MFg” or can they simply log in using User1? We are implementing a single domain with multiple sites and copies of the Global Catalog. We’ll be running mixed mode for a while, but I’d like to know how OUs can affect the login name of users.

TIA,

Bobby