Industrial symbiosis (IS) has gained more attention in the production economics as the pressure on companies increases for the reduction of waste emissions as well as of primary resources consumption. At present, as the number of IS initiatives increases, many issues about its boundaries, such as the entities involved and the resources shared/exchanged, still continue to remain open. A common classification of IS networks does still not exist.Input-output matching is the most significant tool to analyze IS. To this aim, we adopt an input-output approach for defining an IS network, and for proposing a classification model at the technical level (i.e. physical flow type, number of production chains involved, and network structure).The proposed approach is applied to several existing IS networks to describe as it works. This classification framework can be useful for companies to set strategies and for local government to plan policies.