The Fundamentals of Software-Defined Networking: Towards Intelligent and Flexible Networks is a comprehensive, beginner-friendly guide to Software-Defined Networking (SDN), specifically designed for educators, trainers, and professionals transitioning into modern networking. It offers a structured, standards-based approach that starts with fundamental networking concepts and builds up to advanced SDN architectures. Focusing on accessibility and practical application, it emphasizes open-source tools and real-world labs that can be implemented in low-resource environments. It covers key SDN technologies, including OpenFlow, SD-WAN, SD-LAN, NFV, LISP, and orchestration with Ansible, while integrating Python-based automation and RESTful APIs. Key FeaturesProgressive, beginner-friendly structure ideal for academic and training settings. Hands-on labs using open-source tools such as ONOS, Ryu, Open vSwitch, and Python, plus real-world case studies and projects. Vendor-neutral content with no reliance on proprietary technologies. Supplementary materials, including scripts and emulator images are available online: http://github.com/sadiqui-ali/SDN_resourcesThis book is beneficial for instructors, students, and IT professionals seeking a practical and comprehensive introduction to SDN. Moreover, it is particularly suited for training environments where cost-effective, standards-based learning is essential.