This paper focuses on the use of discrete event simulation techniques for the performance evaluation of FDDI networks interconnected via the ATM WAN. The simulation enables us to obtain relevant data on such performance evaluators as the end-to-end delay and throughput for the traditional TCP/IP traffic over the internetwork. The qualitative analysis provides a basis to formulate the delay and throughput approximations and reveals the intrinsic behaviour of these networks. The detailed network model has been developed in the C language. OPNET, a Unix-based communication systems simulation tool which supports the hierarchical object-oriented model, is used in this simulation. The FDDI-ATM internetwork performance results are presented in the paper.