Network-based virtual worlds allow multiple virtual worlds connected on a network to share information with one another. The development effort required to produce a network-based virtual world is quite large. The BrickNet toolkit simplifies this development by providing the standard facilities required by a wide range of network-based virtual worlds. It provides support for graphical, behavioral and network modeling of virtual worlds in an object-oriented fashion. BrickNet enables graphic objects to be maintained, managed, and used efficiently, and permits objects to be shared by multiple virtual worlds. In this paper, the architecture and implementation of BrickNet are described.