A specification in Enterprise Architecture (EA) requires the modeling of an enterprise across multiple levels, from the markets in which it operates down to the implementation of the IT systems that support its operations. Our goal is the development of a method and of a CAD tool that support such modeling. To achieve our goal, we need an ontology to represent systematically all the systems at the multiple levels identified in an enterprise. We base our ontology on the foundation modeling concepts defined in Part 2 of ISO/ITU Standard "Reference Model of Open Distributed Processing" (RM-ODP). In this paper, we present how multi-level systems can be represented using directly the concepts defined in Part 2 of the RM-ODP. Our modeling approach differs from that defined in Part 3 of the RM-ODP, which focuses on the specification of IT systems in terms of viewpoint models representing the IT system environment and its construction. The benefit of our approach is the capability to model systematically and consistently the multiple systems represented in a company.