This paper presents a network partitioning strategy for the optimal voltage control of Active Distribution Networks (ADNs) by means of Dispersed Energy Storage Systems (DESSs). The proposed partitioning is based on the concept of voltage sensitivity coefficients and is adopted for a decentralized voltage- control strategy specifically developed for radial ADNs. The aim of the partitioning is to decompose the network into quasi- autonomous areas and to limit the information exchange only at the interfacing nodes between adjacent areas. The information exchange relies on Thévenin equivalents to represent the external grids of each cluster/area. The effectiveness of the proposed decentralized control approach is assessed with respect to an equivalent centralized control approach. Such an assessment is carried out using a numerical example referring to IEEE 123 buses distribution test feeder suitably adapted to include stochastic generation and DESSs.