In the cyberoperations community there is a commonly accepted starting point for describing cyberspace as comprising of multiple planes through which information flows. However, the model is not a tool that facilitates planning and executing cyberoperations. Tools do exist in the form of technical cybersecurity ontologies. At the moment the link between technical ontologies, that are the tools of experts, and the operational planning process is limited. These technical ontologies provide automated information that would support operational planning. At the moment cybersecurity experts translate the information that military professionals need, which may cause insufficiencies or distortions in communication or cause inconsistencies in the planning process. This paper presents the ongoing work of developing a model of cyberspace in the form of a core ontology. The ontology describes the flow of digital information between persons and the enabling technology as well as geographical data. It is intended as a tool that supports operational planning and decision-making in and through cyberspace, by enabling automation and reasoning. The model is created using the well-established Constructive Research Approach (CRA) methodology, and is developed on earlier research. CRA consists of six phases in which (1) the problem is defined, (2) an understanding of the topic is generated, (3) a solution (model) is constructed which then is (4) demonstrated. Then the models (5) theoretical connections are presented and the (6) scope of applicability is assessed. The challenges of developing an ontology of cyberspace as part of the third phase of the methodology are in focus. The ontology serves as an operational core ontology, aiming to link cybersecurity domain ontologies to the DOLCE+DnS Ultralite (DUL) foundational ontology. The ontology is based on research in Cyberspace Geography and Cyber Terrain. No earlier attempts at creating a core ontology of cyberspace grounded in a foundational ontology, based on these concepts, were found. Overall, the use of reference ontologies in cyberspace research is scarce and few are grounded in a foundational ontology. The starting point for the ontology is a model of cyberspace comprising of six layers, which are the 1) geographic layer, 2) physical network layer, 3) logical network layer, 4) socio-organizational layer, 5) virtual persona layer and finally the 6) persona layer. The model was complemented with levels describing action and information and partially excluded the outer levels 1 and 6, which were directly linked to the DUL foundational ontology.