This paper examines how object-oriented extensions to the Petri Net formalism provide flexible structuring primitives which can aid the modelling of network protocols. A key benefit is the support for incremental modelling. As a result, a protocol can be modelled as a collection of services, each of which can be expressed as the enhancement of a basic service, in which case, both the structure of the basic service and the nature of the enhancement can be clearly identified. More importantly, the evolution of a protocol through a sequence of standards can be expressed by progressive refinements. The object-oriented extensions are captured in the formalism of Object Petri Nets, with a textual language form referred to as LOOPN++, both of which are introduced in this paper. The incremental modelling capabilities and their benefits are demonstrated for the Z39.50 Protocol for Information Retrieval.