AbstractFlexibility, extensibility and reliability of a computer network are efficiently achieved by a design with hierarchical architecture. In an actual network system the network design should be implemented by modules corresponding to the layers of the hierarchical architecture in order to realize its inherent advantages. Generally, in highly modularized software the problem of overhead in transferring data between modules has to be solved. This paper presents a method of network software design which is easily implemented by a highly modularized software structure and describes implementation of the architecture through separate communication control and data processing in an operating system. These presentations are made by introducing the methods employed in constructing a local computer network called the S&T net. The methods presented in this paper can be applied generally to the design and realization of network software.