NFS is a stateless protocol which means that each request from client to server must contain all of necessary information to understand the request.
NFS is implemented using RPC (remote procedure call) protocol which have multiple procedures that define type of request. For example client send request to server for mount:
mount server:/vol/vol0 /mnt/vol0
This request is coming to PORT 111 of the server where portmapper is listening for RPC calls.Then daemon mountd returned port for registred mount service and allows to do a mount.