Thanks for replying...actually I was going with basics and I almost got the thing.But the confusion is that if we assume N gate connected to the output then the current drown from Rc will be Ni (i = current in single gate connected to output out of N) which will drop voltage at Rc ,reducing the voltage at Y.So Voltage of Y >Voh(lowest High state output of gate) to keep operation normal. So Voh should be given to calculate fanout when output is HIGH .Is my analogy is correct?