Internet of Things (IoT) is the leading topic in the world news today due to its wider potential of applications such as Smarter Cities, Smart Homes, Wearables, Automobile Industries, and others. As IoT is a technology outcome of multidisciplinary research, today’s researchers are in need to develop proof-of-concept (POC) solutions on various aspects of IoT. This chapter highlights the significance of a tool Cooja simulator of Contiki OS in the design and development of IoT networks and solutions. Contiki OS has well-structured functions and modules supporting various design aspects of IoT networks. Especially, the features of communication stack provide extended IoT applications. In the network layer of IoT, a scheme of use of network coding at Packet level can improve the throughput performance of IoT networks. Similarly, the security in the network layer of IoT can be enhanced by developing privacy homomorphism using Contiki OS at rime stack. Body area networks (BAN) with IoT devices can be enhanced with improvised RPL protocol in the network layer for higher performances using Contiki OS at micro-TCP/IP stack. Applications can also be developed by generating new suitable functions of the Contiki tool. In summary, the chapter explores the features of Contiki OS in support and development of IoT networks.