A method is provided for allowing a monitoring system to classify, by entity, IP “accounting” elements passing through routers of a network of a service provider, these entities being arranged in different sites connected to the network. The method includes: each site is associated with an interface of a router to which that site is connected, and during analysis by the monitoring system of an accounting element originating from a router, information relating to the source interface and destination interface contained in this accounting element is used in order to identify the source and destination sites and to classify this accounting element with respect to the correct source or destination entity.