The rapid proliferation of Internet of Things (IoT) devices has transformed modern infrastructure, enabling unprecedented connectivity and data exchange across var ious sectors, including healthcare, manufacturing, and smart cities. However, this massive scale introduces significant security challenges, as traditional network architectures strug gle to accommodate the heterogeneous and resource-constrained nature of IoT devices. This chapter presents a comprehensive network security architecture designed specifically for large-scale IoT infrastructure. By integrating Software-Defined Networking (SDN) and Zero-Trust Architecture (ZTA) principles, the proposed framework ensures dynamic policy enforcement, continuous authentication, and robust threat mitigation. We evalu ate the architecture using the widely recognized CICIoT2023 dataset, demonstrating its efficacy in detecting and neutralizing common IoT threats such as botnets, distributed denial-of-service (DDoS) attacks, spoofing, and Man-in-The-Middle (MITM) attacks. The extensive simulation results highlight a scalable approach to securing IoT networks with out compromising performance, ensuring low latency and high detection accuracy even under substantial device loads.