Over the last two decades several eff orts have been made to provide adequate experimental environments, aiming to ease the development of new network protocols and applications. These environments range from network simulators providing highly controllable evaluation conditions, to live testbeds providing realistic evaluation environment. While these di fferent approaches foster network development in different ways, there is no simple way to gradually transit from one to another, or to combine their strengths to suit particular evaluation needs. We believe that enabling a gradual transition from a pure simulated environment to a pure realistic one, where the researcher can decide which aspects of the environment are realistic and which are controllable, allows to improve network solutions by simplifying the problem analysis and resolution. In this paper, we propose a new network experimentation framework where simulated and real components can be arbitrarily combined to build custom test environments, allowing to refi ne and improve new protocols and applications implementations by gradually increasing the level of realism of the evaluation environment. Moreover, we present a testbed architecture speci fically adapted to support the proposed concept, and discuss the design choices we made based on our previous experience in the area of network testbeds. These choices address key issues in network testbed development, such as ease of experimentation, experiment reproducibility, and testbed federation, to enable scaling the size of experiments beyond what a single testbed would allow.