History of Software-Defined Networking
Another way to understand software-defined networking is to look at where it came from.
One of the earliest examples of this type of idea is early networking systems in which individual hosts were able to talk to a central mainframe. These setups, pioneered in the latter part of the 20th century, are pretty primitive by today's standards – you could describe them as “feedforward” in that the communications only went one way. The process of decoupling the control plane and data plane hadn't been done yet.
Some sources offer the example of the public switched telephone network as one of the first ways that engineers started to separate control and data planes and move toward what eventually became a software-defined networking strategy. Others will note the use of peer-to-peer networks in the 1980s and the rise of micro-computing as another part of the basis for our modern software-defined networking principles. Essentially, scientists started working with the idea that not every routing signal had to be resident in its node – and building that philosophy into diverse systems.
One of the simplest ways to explain this is that over time, the scientific and IT communities were able to slowly innovate how networks function, and where control processes are located. When big vendors like Cisco started to get on board, the whole process of enhancing software-defined networking got supercharged. Again, as cloud computing took off and virtualization became a standard, SDN became a more vibrant discipline in the tech community, within the general category of “software-defined IT.” As cloud computing was born and developed quickly as a revolutionary way to offer nearly any kind of service, network administrators saw the cloud paving the way for the software-defined network models – prior to cloud computing, all of those pieces had to be sourced in the same hardware location, so there wasn't the same focus on innovating some of these types of frameworks. Cloud computing, software as a service and virtualization generally spurred a lot of the design work that led the vanguard of data scientists to consider separating the control plane from the data plane in the precise ways that we're talking about with SDN.