Debian 5 Lenny NFS

Just setup a NFS server on Debian Lenny, then mount the nfs folder and got error message :

support@client03:~$ sudo mount -t nfs 192.168.0.9:/media/HD /media/iso
mount.nfs: 192.168.0.9:/media/HD failed, reason given by server: Permission denied

Usually the problem is file /etc/exports, /etc/hosts.allow and /etc/hosts.deny

For file /etc/exports, usually many people say I can do like this to share the folder for while subnet :
/media/HD 192.168.0.0/24 (ro)

But it doesn't work.

In Debian Lenny, I have to make it as below. It solved the "Permission denied" problem.
/media/HD 192.168.0.0/255.255.255.0 (ro)