As technology advances, the metal width is decreasing with the length increasing, making the resistance along the power line increase substantially. Together with the nonlinear scaling of the threshold voltage that makes the ratio of the threshold voltage to the supply voltage rise, the voltage (IR) drop become a serious problem in modern VLSI design. Traditional power/ground (P/G) network analysis methods are typically very computationally expensive and thus not feasible to be integrated into floorplanning. To make the integration of the P/G analysis with floorplanning feasible, we need a very efficient, yet sufficiently accurate analysis method. In this paper, we present the methods for the fast analysis of the P/G networks at the floorplanning stage and integrate our analyzer into a commercial tool to develop a power integrity (IR drop) driven design methodology. Experimental results based on three real-world circuit designs show that our P/G network analyzer is accurate enough and very efficient.