We present the IP Network Design Workbench which supports collaborative network design and simulation by a group of experts and customers. This system is based on a tangible user interface platform called "Sensetable," which can wirelessly detect the location and orientation of physical pucks. Using this system, users can directly manipulate network topologies, control parameters of nodes and links using physical pucks on the sensing table, and simultaneously see the simulation results projected onto the table in real-time. Keywords tangible user interface, network design, network simulation, computer supported collaborative work