An end to end IoT system includes various networks in an IoT system, type of devices and the applications running in those networks. Constrained node network is deployed as an edge network in an IoT system. There are various classes of constrained nodes based on the memory constraints, network stack and security support. This constrained node network comprises mainly of sensors, actuators and gateways. Sensors and actuators interacts with the external world and communicates to other networks through gateways. Constrained networks cannot use conventional TCP/IP stack and need a new network stack because of its resource constraint nature. These protocols depend on the link layer technologies used. Most common link layer technologies are Zigbee, Thread, BLE & Z-Wave. The firmware design relies heavily on which type of link layer technology is used in a constrained node. Among the various firmware, thread is slowly becoming the de-facto technology for constrained nodes.This blog presents a brief introduction about IoT, its building blocks and then focuses on the constrained node networks, their characteristics, protocols used and the recommended firmware.