If you have never studied at the University of Vienna, you have to preregister at https://erstanmeldung.univie.ac.at
If you're already studying at the University of Vienna and you want to study further study programmes or if you want to change your studies, you have to register at UNIVISonline (https://univis.univie.ac.at). You also have to register at UNIVISonline if you lost the admission at the University of Vienna and if you want to re-apply. You need your u:net passwort in order to be able to login at UNIVISonline: www.univie.ac.at/ZID/unet/
Please notice: you can only be admitted to studies for which you did the preregistration in time!