Assign a reference node:

The Node voltage analysis establishes the voltages at the nodes in a circuit with respect to a reference at which the voltage is stated to be zero. Generally the negative terminal of the supply or a terminal shared by supplies in the circuits with numerous voltage supplies is selected as the reference, though any other terminal could be selected as reference also. Numerous names are used interchangeably for the reference node involving common node and ground node.

                                                  Figure: Example circuit with reference node

The fundamental node voltage analysis method explained here supposes that all voltage sources have one terminal associated to the reference node. A slightly modified method is employed for circuits having floating voltage sources (that is, voltage sources with neither terminal tied to the reference node).