L.0 language is a new parallel, high-level, executable specification language created at Bellcore for the design and implementation of software systems with inherent concurrency such as communications protocols, services, and networks. L.0 was explicitly designed to express coordination, simultaneity, and the hierarchical composition of systems from component subsystems.L.0 has been used to prototype communications protocols and services and to study network architectures and switching systems. This paper discusses the initial application of L.0: the prototyping of a large portion of an experimental data communication services network architecture being studied by Bellcore.