For discrete time nonlinear systems satisfying an exponential or finite time controllability assumption, we present an analytical formula for a suboptimality estimate for model predictive control schemes without stabilizing terminal constraints. Based on our formula, we perform a detailed analysis of the impact of the optimization horizon and the possibly time varying control horizon on stability and performance of the closed loop. Key words. nonlinear model predictive control, suboptimality, stability, controllability, networked control systems AMS subject classifications. 49N35, 93D15, 93B05