[CentOS] NFS mount problem

Hi,guys:

   I am trying to connect to a NFS-filesystem on a CentOS 5.3 i386  guest
system. The host system is CentOS 5.4 x86_64.
 I set up the NFS server correctly and rpcinfo is also ok  inside the guest
system show.

The problem is, I can't connect to, or see the open port from the outside. I
use the Host-Only  network setup with VirtualBox 3.1.0,
the ip address in the guest system is 192.168.56.101 . and the host system
ip address is 192.168.7.67

When I mount the directory from the guest system via NFS, the result is the
following :
"mount : 192.168.56.101:/xx  failed , reason given by server:Permission
denied"

 I hope one of you can give me a hint in which direction I should continue
my efforts.

Thanks in advance,,,
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