Abstract: Active networking techniques embed computational capabilities into conventional networks thereby massively increasing the complexity and customization of the computations that are performed with a network. In depth studies of these large and complex networks that are still in their nascent stages cannot be effectively performed using analytical methods. Hence, discrete event simulation techniques are the only viable means to study and analyze active networking architectures. Furthermore, customized and flexible tools are required to for the analysis of active networks using simulation. This paper describes an integrated environment for the modeling and parallel simulation of active networks called Active Networks Simulation Environment (or ANSE). ANSE utilizes the Time Warp synchronized kernel of WARPED (a general purpose discrete event simulation kernel) to enable parallel simulation of active network models. ANSE also includes complete support for the modeling and simulation of active networks based on PLAN (Packet Language for Active Networks). This paper presents the issues involved in the design and development of ANSE. The Application Programming Interface (API) of ANSE is presented along with the issues involved in utilizing it to develop support for PLAN based active networks. The paper also presents some results obtained from the several experiments conducted to evaluate the effectiveness of ANSE. Our studies indicate that ANSE provides an effective environment for modeling and simulation of large scale active networks.