When you log in to AC or FS (through the command line) for the first time after a new installation, a default domain (named default) will be the only existing domain. This domain is provided to allow you to log in to AC or FS for the first time.
Once you log in, create a valid domain that is capable of holding administrators and then create an administrator in that domain. For security, we recommend you change the password of the administrator account on the default domain. Log in to the default domain as the default administrator. Then change the password for the administrator account.