It is expected that the Internet technologies and protocols will be introduced into field networks that connect controllers and field devices such as sensors, actuators, and so on. Foundation fieldbus high speed Ethernet (FF-HSE) is one of the control protocols which built on the Internet protocol (IP). We discuss the use of FF-HSE in the environment that the field network is composed of multiple segments. The control protocols uses IP multicast because they originate from the bus architecture. However, there are problems of the scalability in existing technology. We propose a multicast packet tunneling system MMT (multicast mediator tunnel) against such problems.