Internet is playing indispensable role in our daily lives. With recent advancement of communication technologies, Internet of Things (IoT) became vital part of human life. IoT devices may be easily compromised and incapable of defending & securing themselves due to resource constrained nature. Since, the integration of devices with resource rich pool such as cloud is required. The ability of current cloud model is insufficient to handle requirements of delay sensitive IoT applications. Cloud-IoT integration model does not support the features e.g. geographical distribution, low latency and location awareness etc. that features are necessary for some IoT applications includingtraffic light management, smart healthcare management and smart home energy management. Fog computing is still an evolving architecture that demands more research. Security is one of the major issue in fog computing. In this paper, we proposed an anonymous mutual authentication scheme based on ECC for fog enabled IoT environment. The proposed protocol ensures device anonymity and achieves mutual authentication between IoT device and fog node with the help of trusted third party (TTP) called centralized authentication protocol. Security analysis of proposed authentication protocol shows that the protocol is vigorous against various cryptographic attacks. The performance analysis shows that the protocol is efficient and computationally feasible in terms of storage and communication overhead for resource constrained environment.