ADIOP is an application for Automated Diagnosis of InterOperability Problems. Interoperability testing involves checking the degree of compatibility between two networking devices that implement the same protocol. In ADIOP, each interoperability test case is first modeled as a Constraint Satisfaction Problem. Object-Oriented Programming is used to implement ADIOP. In this paper, we present the modeling language we use in ADIOP and how it allows the user to easily and efficiently create test cases and use them for diagnosis. The specific domain of application is interoperability testing of protocols in ATM (Asynchronous Transfer Mode) networks.