Norton’s Theorem

Norton’s Theorem  states that any linear, active, bilateral network, considered across one of its ports, can be replaced by an equivalent current source (Norton’s current source) and an equivalent shunt admittance (Norton’s Admittance).Since the two sides are identical, they must be true for all conditions. Thus if we compare the current through the port in each case under short circuit conditions, and measure the input admittance of the network with the sources removed (voltage sources short-circuited and current sources open-circuited), then 

Inorton =  Isc

Ynorton =  Yin