Linux – mount.nfs: access denied by server while mounting in Ubuntu Machine

I have three machines –

machineA    10.108.24.132
machineB    10.108.24.133
machineC    10.108.24.134

and all those machines have Ubuntu 12.04 installed in it and I have root access to all those three machines.

Now I am supposed to do below things in my above machines –

Create mount point /opt/exhibitor/conf
Mount the directory in all servers.
 sudo mount <NFS-SERVER>:/opt/exhibitor/conf /opt/exhibitor/conf/

I have already created /opt/exhibitor/conf directory in all those three machines as mentioned above.

Now I am trying to create a Mount Point on all those three machines. So I followed the below process –

Install NFS support files and NFS kernel server

$ sudo apt-get install nfs-common nfs-kernel-server

Create the shared directory

$ mkdir /opt/exhibitor/conf/

Edited the /etc/exports and added the entry like this:

# /etc/exports: the access control list for filesystems which may be exported
#               to NFS clients.  See exports(5).
#
# Example for NFSv2 and NFSv3:
# /srv/homes       hostname1(rw,sync,no_subtree_check) hostname2(ro,sync,no_subtree_check)
#
# Example for NFSv4:
# /srv/nfs4        gss/krb5i(rw,sync,fsid=0,crossmnt,no_subtree_check)
# /srv/nfs4/homes  gss/krb5i(rw,sync,no_subtree_check)
#
/opt/exhibitor/conf/     10.108.24.*(rw)

Run exportfs

root@machineA:/# exportfs -rv
exportfs: /etc/exports [1]: Neither 'subtree_check' or 'no_subtree_check' specified for export "10.108.24.*:/opt/exhibitor/conf/".
  Assuming default behaviour ('no_subtree_check').
  NOTE: this default has changed since nfs-utils version 1.0.x

exporting 10.108.24.*:/opt/exhibitor/conf

Now I did showmount on machineA

root@machineA:/# showmount -e 10.108.24.132
Export list for 10.108.24.132:
/opt/exhibitor/conf 10.108.24.*

And now when I did this, I am getting an error –

root@machineA:/# sudo mount -t nfs 10.108.24.132:/opt/exhibitor/conf /opt/exhibitor/conf/
mount.nfs: access denied by server while mounting 10.108.24.132:/opt/exhibitor/conf

Any idea what wrong I am doing here?