Co-design modeling is considered key toward handling the complexity and scale of network systems. The ability to separately specify the software and hardware aspects of computer network systems offers new capabilities beyond what is supported in modeling frameworks and tools such as NS-2 and OPNET. The DEVS/DOC simulation environment supports logical co-design specification based on the Distributed Object Computing (DOC) abstract model. To overcome DEVS/DOC's lack of support for visual and persistent modeling, this paper presents SESM/DOC, a novel approach, which is based on the Scaleable Entity Structure Modeler (SESM), a component-based modeling framework. This approach supports logical, visual and persistent modeling. Modelers can develop software and hardware models separately and systematically integrate them to specify a family of computer network system designs. This paper details the SESM/DOC co-design modeling approach with the help of a search engine system example, and presents a discussion for future research directions.