Some services, mainly SUN's NIS and NFS services, don't use a static port like most services, but instead register themselves via RPC. To connect to an NFS server you first ask RPC which port to find it on. It may even be the case that the service isn't available until you request it via RPC. If you want to list the registered RPC services run rpcinfo:
[swhite@fishy swhite]$ rpcinfo -p localhost
program vers proto port
100000 2 tcp 111 portmapper
100000 2 udp 111 portmapper
100004 2 udp 644 ypserv
100004 1 udp 644 ypserv
100004 2 tcp 647 ypserv
100004 1 tcp 647 ypserv
100007 2 udp 663 ypbind
100007 1 udp 663 ypbind
100007 2 tcp 666 ypbind
100007 1 tcp 666 ypbind
100005 1 udp 635 mountd
100005 2 udp 635 mountd
100005 1 tcp 635 mountd
100005 2 tcp 635 mountd
100003 2 udp 2049 nfs
100003 2 tcp 2049 nfs
100009 1 udp 873 yppasswdd