Due to the bandwidth constraint and unreliable transmission of network, data packet dropout exists inevitably in networked control systems (NCSs). Networked three tank system with the controller designed without having the network in mind is analyzed and conclusions are drawn that network should be considered when designing NCSs. Next, NCSs with arbitrary but finite data packet dropout is modeled as switched linear system and sufficient conditions are presented on the stability of NCSs. The corresponding state feedback controller can be constructed by using the feasible solutions of a group of linear matrix inequalities. Simulation results for the three tank system are given, which prove the validity of the design method.