I went ahead and chose the Linux Red Hat version 4 for the operating system although there is a CentOS running (this is another topic in the forum). In this configuration I chose the head node to be a compuatation node. Since the head node has 4 processors, the mpirun -np x mpihello works fine for upto 4. After that (it needs to communicate with other nodes) it just hangs. In my last installation I had not used the head node for computation and I could not do mpirun from the head node at all.
Now if I log on to another node I can do an mpirun -np x mpihello where x works fine upto the max number of processors on cluster.
So why is it that if mpirun -np x mpihello is invoked from the head node it cannot contact other nodes but vice versa works? (I am using rsh as the communication protocol between nodes)
Also note that something like mpirun -np 10 hostname works fine. So the problem appears to be when I make the MPI calls in mpihello.