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A number of methods exist that can be used to create simulation models for measuring the performance of computer networks. The most commonly used method is packet level simulation, which models the detailed behavior of every packet in the network, and results in a highly accurate picture of overall network behavior. A less frequently used, but sometimes more computationally efficient, method is the fluid model approach. In this method, aggregations of flows are modeled as fluid flowing through pipes, and queues are modeled as fixed capacity buckets. The buckets are connected via pipes, where the maximum allowable flow rate of fluid in the pipes represents the bandwidth of the communication links being modeled. Fluid models generally result in a less accurate picture of the network's behavior since they rely on aggregation of flows and ignore actions specific to individual flows. We introduce a new hybrid simulation environment that leverages the strong points of each of these two modeling methods. Our hybrid method uses fluid models to represent aggregations of flows for which less detail is required, and packet models to represent individual flows for which more detail is needed. The result is a computationally efficient simulation model that results in a high level of accuracy and detail in some of the flows, while abstracting away details of other flows. We show a computational speedup of more than twenty in some cases, with little reduction in accuracy of the simulation results.