Active networks represent a significant step in the evolution of packet-switched networks, from traditional packet-forwarding engines to more general functionality supporting dynamic control and modification of network behavior. However, the phrase "active network" means different things to different people. This article introduces a model and nomenclature for talking about active networks, describes some possible approaches in terms of that nomenclature, and presents various aspects of the architecture being developed in the DARPA-funded active networks program. Potential applications of active networks are highlighted, along with some of the challenges that must be overcome to make them a reality.