The Army Research Laboratory (ARL) is conducting a variety of research programs in mobile communications and networking to support the digital battlefield of the future. The U.S. Army will require techniques such as mobile ad hoc networks to form and maintain wireless network connectivity in this dynamic battlefield environment. It will also require gateways that can bridge ad hoc networks to higher-echelon Internet-based backbone networks via a flexible combination of wireless, symmetric and asymmetric satellite, cellular, and wired network connections, as well as via legacy combat radio systems such as the Single Channel Ground and Airborne Radio System (SINCGARS). ARL has designed and built a mobile communications and networking Testbed on a High Mobility Multipurpose Wheeled Vehicle (HMMWV) to support our mobile wireless networking program. The Testbed is designed to combine various computing, networking, and communication techniques and technologies on a mobile platform relevant to the Army digital battlefield. With wire/wireless and symmetric/asymmetric WAN connections, the Testbed provides Internet gateway capabilities by direct or indirect routing using IP masquerade techniques. IP tunneling techniques are also used to implement mobile IP LANs with full connectivity to backbone networks via the Testbed. This paper discusses ARL research and experimentation in mobile gateway techniques such as IP masquerading and IP tunneling and describes the use of these techniques in the Testbed to provide mobile gateway functionality under actual field conditions.