Traditional IP networks are complex and hard to manage. The vertical integration of the infrastructure, with the control and data planes tightly coupled in network equipment, makes it a challenging task to build and maintain efficient networks in an era of cloud computing. Software-Defined Networking (SDN) breaks this coupling by segregating network control from routers and switches and by logically centralizing it in an external entity that resides in commodity servers. This way, SDN provides the flexibility required to dynamically program the network, promoting the “softwarization” of networking. In this article we introduce this new paradigm and show how it breaks the status quo in networking. We present the most relevant building blocks of the infrastructure and discuss how SDN is leading to a horizontal industry based on programmable and open components. We pay particular attention to use cases that demonstrate how IT companies such as Google, Microsoft, and VMware are embracing SDN to operate efficient networks and offer innovative networking services.