Abstract: A software object model for modelling resources (resource instances (RI) and resource types (RT)) in a network management system. The model comprises object instances (OI) and object classes (OC). Each class comprises fragments (FG) of data and/or program code and is a generalization of one or more object instances (OI). The model models each resource type (RT) by an object class (OC) and assigns to the class a class version identifier (CVI) which indicates at least some capabilities of the resource (RI, RT) and a collection of the fragments (FG) included in the class (OC) of the version in question. The model further models each resource instance (RI) by an object instance (OI) of the corresponding class, and assigns to the instance (OI) an object instance identifier (OII) and an instance version identifier (IVI) which indicate the version of the class (OC) used for modelling the resource type (RT) in question.