Hannover has:

-Crowne Plaza Schweizerhof
-ArabellaSheraton Pelikan

I have been at CEBIT at least 15 times and never got a hotel room! All fully booked.
I had to stay at private places, which is customary during big fairs. Look under "Zimmervermittlung".
May be your company, your supplier (IBM, HP other) has reserved hotel rooms for VIPs.
BTW, Cebit is a "zoo"!

The location is not so important: In the morning most of the trains/trams go in the direction Messe/Fair. In the evening vis-versa.