I want to configure my hosts.allow file to restrict access to sshd to my school's computer labs, which are in a range, for example, 12.34.*

The way I was told to do this was changing the hosts.allow to read:
When I do this, nobody can access sshd. However, I change it back to reading:
sshd: ALL
and after that it's happy and lets anyone connect. I have also tried entering the individual IP in there, which also doesn't seem to work.

I don't understand why it only works when sshd: ALL is in there. Any ideas from anyone?