Book excerpt: Software Defined Networking is revolutionizing the networking world. While the industry transitions to a software-centric architecture, a clear definition of SDN remains murky at best. This book clarifies the current industry confusion about what SDN is, why it's important, and most importantly the protocols and use cases that define SDN. OpenFlow (OF) is a critical piece of the SDN puzzle. While SDN solutions exist that do not require OF, it is undeniable that OF helped spur the innovation in SDN. The history of OF, its current and future status, and the associated use cases will be explained in detail in this book. Lastly, the book attempts to lay out SDN deployments that are real and current today, and apply practicality to the vast world of SDN architectures.