This paper describes transparency and system connection for future heterogeneous haptic network systems, e.g. bilateral teleoperation systems which include communication networks with different capacities and haptic devices with different degrees of freedom (DOF). This paper also proposes a controller design method of bilateral control systems in the heterogeneous environment. A one-dimensional master-slave haptic display system is taken as an example in this research. Firstly, haptic information, i.e. the position of each haptic device and the reaction force exerted on each haptic device, is transformed into environmental modes by means of mode transformation. Then the haptic information in the modal space is transmitted between master and slave haptic devices. The controller is designed so as to optimize mode-space transparency. Transparent bilateral control in the modal space is achieved even when the system connection changes, e.g. the capacity of communication network or the DOF of haptic devices changes. The proposed controller design method is validated by experimental results.