A modified gravity model is used to describe the flows of commuters between a central city and concentric zones around it. A multiplier-type model of a central-place system (after Christaller) is then used to calculate the number of residents for service centers at each level of the hierarchy within the system. The rank-size rule (Zipf's formula) is also considered as a criterion of a true system of urban places. Results are presented for the urban network of the Ukraine.