In this paper we describe the use of the OPNET simulation tool for analysis of packet data networks. The motivation is to provide knowledge on packet level networks: Fiber Distributed Data Interface and Network intrusion simulation. FDDI protocol is examined by varying network parameters in two network configurations. Geographic distribution of servers and workstations in different buildings by capitalizing on the long-distance capability also saves money by eliminating the necessity for moving equipment to a single location provides high utilization on the FDDI LAN, thus lowering the cost of transporting data. We discussed about OPNET process model and its performance and effect on the traffic patterns in an ATM network. In this paper we also report studies of simulation efficiency and network performance of simulated network using firewall.