Smart City solutions are currently based on multiple architectures, standards and platforms, which have led to a highly fragmented landscape. In order to allow cities to share data across systems and coordinate processes across domains, it is essential to break these silos. A way to achieve the purpose is sensor virtualization, discovery and data restitution. In this paper, a federation of FIT IoT-LAB within OpenIoT is presented. OpenIoT is a middleware that enables the collection of data streams from multiple heterogeneous geographically dispersed data sources, as well as their semantic unification and streaming with a cloud infrastructure. Future Internet of Things IoT-LAB (FIT IoT-LAB) provides a very large scale infrastructure facility suitable for testing small wireless sensor devices and heterogeneous communicating objects. The integration proposed represents a way to reduce the gap existing in the IoT fragmentation, and, moreover, allows users to develop smart city applications by interacting directly with sensors at different layers. We illustrate it trough a basic temperature monitoring application to show its efficiency.