A Knowledge Based Network is a type of ontological content based network. As Knowledge Based Networks scale, semantic interoperability becomes an important issue since larger populations of users result in more heterogeneity in the content of messages. This paper examines the content heterogeneity problem in a KBN and proposes a mapping service scheme for distributed and heterogeneous knowledge-based applications. It compares a number of strategies that use pre-existing semantic mapping information stored in KBN routers. Evaluation results show that this scheme can effectively solve the heterogeneity problem.