The technological breakthroughs in IoT improve the life quality of humans day by day. The evolution of smart IoT applications tends to the development of several protocols. The protocols are developed to meet the expectations of various scenarios and applications. Different routing protocols are invented to meet the specific characteristics of various IoT applications. Some of the familiar IoT applications are HyperText Transfer (HTT), Advanced Message Queuing (AMQ), Blue tooth and Bluetooth Low Energy (BLE), Cellular, Constrained Application Protocol (CoAP), Data Distribution Service (DDS), Extensible Messaging and Presence (EMP), Lightweight Machine to Machine (LWM2M), Long-range and LoRaWAN, MQTT, Wi-Fi, Zigbee, and Wave. The IoT protocols define rules for connecting devices over a network. The low power and low processing capability of IoT protocols can fit with the requirements of tiny sensor devices. However, some of the IoT applications require greater complexity for ensuring communication security and reliability, and in such cases, the protocols are unsuitable for IoT devices that have limited memory. To overcome such issues, recent research works on the design of IoT protocols include artificial intelligence, machine learning, and game models.