An onboard electrical system for a motor vehicle is provided having a primary power supply network and a secondary power supply network electrically connected to the primary power supply network via a DC-DC converter, wherein the primary power supply network and the secondary power supply network are connected to one another via a blocking diode of the DC-DC converter and the blocking diode has a forward direction thereof oriented from the primary power supply network toward the secondary power supply network, and wherein an insulation monitoring device having a measuring resistor is associated with the secondary power supply network for determining an insulation fault. It is provided that an additional resistor is connected in parallel with the blocking diode for bridging thereof, such that an insulation fault common to the primary power supply network and the secondary power supply network can be determined by the insulation monitoring device. A related method for operating an onboard electrical system for a motor vehicle is also provided.