Research papers in routing protocols for the Internet of Things (IoT) focus on the design, optimization, and evaluation of mechanisms that enable efficient, reliable, and scalable data transmission across heterogeneous and resource-constrained IoT networks. These works analyze a wide spectrum of routing approaches, including proactive, reactive, and hybrid protocols, each tailored to balance energy efficiency, latency, reliability, and scalability. Standardized protocols such as RPL (Routing Protocol for Low-Power and Lossy Networks), LOADng, and DYMO are widely studied for wireless sensor networks and 6LoWPAN-based IoT, while geographic, hierarchical, and cluster-based routing models are explored to reduce control overhead and extend network lifetime. Advanced research introduces QoS-aware routing, multipath routing, cross-layer optimization, cognitive routing, and trust-based protocols that address challenges like mobility, congestion, energy depletion, and security threats. Security-aware routing is a prominent focus, with studies proposing lightweight encryption, authentication, intrusion detection, and blockchain-based trust models to defend against attacks such as sinkhole, wormhole, or selective forwarding. Emerging works also integrate artificial intelligence and machine learning techniques to enable adaptive, predictive, and context-aware routing capable of handling dynamic topologies and diverse traffic patterns. Application-driven research demonstrates the deployment of routing protocols in smart healthcare systems, smart grids, industrial IoT, precision agriculture, and vehicular IoT, where requirements such as ultra-low latency, reliability, and fault tolerance are critical. Collectively, this literature highlights how innovative routing protocols form the backbone of IoT communication, addressing the persistent challenges of energy efficiency, scalability, interoperability, and resilience, while shaping the evolution of next-generation IoT networks empowered by 5G, 6G, and beyond.