We propose a network design model for flat networks using hybrid optical/electronic equipments. We describe a set of network components that can be taken into account by this model. We formulate the network design problem and illustrate its difficulties. We then introduce a simpler model for which solutions can be computed easily. Using this simple model, we give numerical results that illustrate the impact of the cost of optics and the cost of electronics on the resource utilization and on the optimal topology of the logical network. One of the main results is that degree constraints on the switches can have a significant impact on the network cost. Another important result is that one cannot rely on intuition to find a good tradeoff between optics and electronics.