## 1 Introduction  
Recent advances in wireless communications and electronics have enabled the development of low-cost, low-power, multifunctional sensor nodes that are small in size and communicate untethered over short distances. A sensor network consists of a large number of sensor nodes that are densely deployed either inside the phenomenon of interest or very close to it [1; 2; 3; 4; 5]. Due to the rapid advancement of electronics, tiny sensor nodes are capable of supporting IP protocol stack. 6 LoWPAN facilitates the IPv6 communication over low power and low cost sensor nodes [5; 6; 7]. In the past, applications of sensor networks were thought to be very specific. The communication protocols of sensor networks were also very simple and straightforward. Some researchers were even against the use of the internetworking concept in WSNs for different reasons such as the resource constraints for layered architecture, the problems of configuring large numbers of devices, the essence of sensor nodes' distinct identity, _etc.,_ but with the advent of the Internet of Things and federated IP-WSNs, this demand is going to be blurred. The huge numbers of IPv6 addresses, the necessity for end to end communication and advances in micro-electronics have changed the concepts of the research community. Now a tiny sensor node can hold a compatible TCP/IP protocol stack, so we can now think of using the concept of internetworking protocols in IP-WSNs. We can easily think of providing IPv6 addresses to individual sensor nodes since it provides around 6 \(\times\) 10\({}^{23}\) addresses per square meter of the Earth's surface. IP-enabled sensor nodes have opened the door for further research into advanced and distributed applications in sensor networks [5]. The limitations of resource constraints of a tiny sensor node could not stop the advancement of research in the field of sensor networks. Now a tiny sensor node can also be identified by an individual IP address [6; 7]. Recently, network virtualization has created a resonance among the network based research community. The concept of sensor virtualization has also attracted a great deal of attention from industry and academia [8; 9; 10]. Virtualization on sensor networks (VSN) can be defined as the separation of the function for the traditional wireless sensor network (WSN) service provider into two parts: sensor infrastructure provider (SInP) that manages the physical sensor infrastructure, and sensor virtualization network service provider (SVNSP) that develops the VSN by aggregating resources from multiple SInPs and offer services to the application level users (ALU). The WSN virtualization renaissance has been caused mainly from the realization that most of the sensor nodes in a WSN remain idle for most of the time. Sensor network virtualization is one of the best ways to utilize the physical sensor node. Virtualization of sensor networks can provide a platform upon which novel sensor network architectures can be built, experimented and evaluated [11; 12; 13; 14; 15; 16; 17]. In addition, virtualization in WSNs is expected to provide a clean separation of services and infrastructure and facilitate new ways of doing business by allowing the trading of sensor network resources among multiple service providers and application level users [18; 19; 20]. This type of virtual sensor environment can be ensured from the coexisting heterogeneous WSN architectures that are free from the limitations of existing multi-vendor sensor networks [21]. The importance of sensor virtualization is manifold in this age of worldwide economic recession. VSN can provide cost effective and green technology solutions to design smart houses and cities. In this paper we survey the virtualization of wireless sensor network, discuss the challenges and opportunities.