[CentOS] NFS/NIS and firewalls

I have a few C5 machines on an isolated LAN that connect to a RHEL5 
server via NFS and NIS for authentication.   I discovered that one of the 
C5 workstations worked fine for NFS exporting, but refused to 
collaborate with the EL5 server for NIS user authentication.

I had successfully connected other systems to this server without issue, 
but this machine was finicky.

I had initially enabled the EL5 firewall, then later disabled it, 
including selecting --disable-firewall.  Still, this one C5 
workstation wouldn't cooperate for user authentication.

Then, as a bit of an experiment, I opted to visit the EL5 services and 
manually highlight IPtables, clicked STOP, and tried the verification 
again.  This time, the C5 system got the NIS data.


Now, if I want to enable a firewall on all machines -

As a server, EL5 does have an option to select NFS services be run on 
specific ports.   How do I configure the C5 clients to also communicate on 
those ports, thus allowing full NIS/NFS user authentication and directory 
exporting, all the while with built-in firewall protection on all systems?

These are all out-of-box setups, with no updates, and full package 
installs from the install media.

As time goes on, I will migrate to the OpenLDAP world, but I haven't had 
the opportunity to experiment with that just yet.

Thanks.

Scott