With the advancement in mobile computing and wireless communications, a new paradigm called Internet of Things is generating a lot of research interest and the industrial revolution. The increasing interest for this paradigm has resulted in the large-scale deployment of Low power and Lossy Networks (LLN), such as wireless sensor networks and home automation systems. These networks are typically composed of many embedded devices with limited power, memory, and processing resources interconnected by a variety of links, such as IEEE 802.15.4 or low-power Wi-Fi. These networks have a wide scope of applications such as industrial monitoring, connected home, healthcare, environmental monitoring, urban sensor networks, energy management, and assets tracking etc.RFC 7228. In order to address the specific properties and constraints of these networks, RPL (Routing Protocol for low power Lossy network) has been developed by the IETF working group [ROLL WG]. RPL is a lightweight, rank based routing protocol. However, this routing protocol is exposed to various attacks which can significantly impact the network resources and its performance. This paper presents an elaborate classification of the possible attacks against RPL in IoT network. Further, we have analyzed and compared the severity of these attacks.