A node is usually labelled as the reference node also called ground and the other node voltages are defined with respect to this point. The reference node has a potential of 0 V by definition.
A circuit with four nodes solved using the Nodal Analysis. Step-by-step solution is provided. The circuit has three current sources and three resistors.
A simple DC resistive circuit with three resistors and two current sources are solved by the Nodal Analysis and power of one current source is determined.