Interconnections of several nonlinear systems with inputs are considered in this paper. Each system is assumed to be input-to-state stable (ISS). However an interconnection of such systems is not stable in general. We provide a stability condition for interconnections of such systems. Some interpretations of this condition are given. Moreover we show how an ISS-Lyapunov function for such a network can be constructed explicitly if this condition is satisfied. Further we give the idea of numerical verification of the small gain condition for networks.