I'm curious but recently I was troubleshooting some iptables rules to allow nfs clients access to my nfs server. What was strange was that I setup a tcpdump session on my nfs server so that I can see which ports were being requested. I ran several tcpdump sessions with the following filters in place.

tcpdump -vv src ip_of_client and dst _ip_of_client
tcpdump -vv src hostname_of_client and dst hostname_of_client

However, the only packet I ever saw come over the wire to me was the client host asking for a arp resolution.

Anyhow, I finally just ran 'rcpinfo -p' and added those ports to my iptables rules and it worked great. However, I would like to understand how nfs works in case I need to troubleshoot it in the future. I do understand that nfs uses portmappers, would this explain the behavior?

Chap 18 here http://www.linuxtopia.org/online_boo...ion/index.html is very informative. Ignore the fact it's an RH manual.
BTW, as of nfsv4, the various 'extra/helper' protocols like portmapper are built-in to nfs. There's a note about that in that chapter.