Today there is a trend of the Internet of Things (IoT) where many objects are connected to the internet. In future number of objects connected to the internet will be more as compared to people in the world. So providing security to IoT devices is one of the ongoing research issues. Inserting security in IoT devices is challenging because maximum devices involved in IoT are resource constrained in terms of battery power and memory size. In IoT, nodes communicate using insecure internet which makes network exposed to various attacks. RPL (Routing protocol for low power and lossy network) is the protocol specially designed for IoT network is very prone to various security attacks. The proposed work in this paper is an implementation of an intrusion detection system (IDS) for Wormhole attack and attacker. Wormhole attack is one of the most severe attacks taking place at 6LoWPAN adaption layer of RPL network. In this type of attack, a pair of attacker nodes forms a tunnel between two nodes as if they are directly connected to each other to misguide network traffic. The proposed IDS is implemented in Contiki OS, using Cooja Simulator. We have used received signal strength indicator (RSSI) to identify the attack and attacker node.