Abstract: Interconnecting heterogeneous devices, that is, devices with varying capabilities, has raised new challenges in the design of network protocols. This paper describes the design of the Flexible Interconnecting Protocol, or FLIP, whose goal is to interconnect heterogeneous devices. FLIP is a flexible protocol that addresses the needs of heterogeneous networks: it incurs little overhead when run by simple devices, while still providing a range of functions that can be performed by more sophisticated devices. We describe a simplified implementation of FLIP under Linux. We also conducted a preliminary evaluation of FLIP's overhead and functionality in the context of IP (IPv4 and IPv6) and sensor network environments. FLIP incurs reasonably low overhead when providing IPv4 and IPv6 functionality (1 and 3 bytes respectively), yet it does particularly well in the case of small payloads. When compared to a sensor-specific protocol, FLIP incurs a small overhead increase while still providing full protocol functionality.