An on-board electrical network (4) of a motor vehicle (2) has a first voltage circuit (I) and a second voltage circuit (II), wherein the first voltage circuit has a first operating voltage higher than a second operating voltage in the second voltage circuit. The first voltage circuit is connected to the second voltage circuit via a DC voltage converter (8). The first voltage circuit has a battery (10) and the second voltage circuit has a main battery (12) and an auxiliary voltage source (14). By means of a first switching element (16) and a second switching element (18) at least one of transmission control unit (20) and/or integrated hybrid controller (22) are supplied with electrical energy from the main battery (12) and/or the auxiliary voltage source (14). Control unit (20) and/or hybrid controller (22) can be selectably disconnected from either the main battery (12) and/or the auxiliary voltage source (14).