Defining virtualization and software-defined networking: What's the difference?

Telecoms are constantly on the search for new ways to improve the delivery of services and improve customers’ experience. Recently, many of these efforts have been focused on the technology being utilized in telco infrastructures, highlighting the use of network function virtualization and software-defined networking.
As these two advanced systems increasingly become the focus of telecom discussion, one main question springs to mind: What is the difference?
Queries of this kind are not uncommon, or unfound, as many do not understand the particulars of each technology, let alone the relationship that connects them. However, comprehending the differences and connections between SDN and NFV is essential to ensure that they are implemented and leveraged in the most effective manner.
How SDN and NFV got their start: Solutions to address different issues 
One way to define the difference between these technologies is to examine the ways and environments in which they were developed. According to SDNCentral contributor Prayson Pate, SDN was created by researchers on campus networks who discovered the need for a better approach to testing out new techniques. Each time an adjustment was needed, researchers had to fundamentally change the software of each network device. The need for a centralized solution helped to shape the characteristics that make up today’s SDN technologies, including a separation of control and forwarding capabilities, concentrated management and the ability to program new network behaviors within an interface.
“[Researchers] came up with the idea of making the behavior of the network programming devices programmable, and allowing them to be controlled by a central element,” Pate wrote.
On the other hand, NFV was created by a collection of service providers to address issues connected with the launch of new network services. Before NFV, telcos’ networks were made up of a large number and range of different hardware components. However, when new services were to be launched, telecoms had to add new hardware to support these processes. Groups quickly realized that this created a number of problems, including rising energy costs, increasing capital investments and the need for skilled individuals to deploy and maintain the hardware.
However, Pate noted that an original white​ paper first outlining NFV technology provides the answer to solving these issues.
“Network functions virtualization aims to address these problems by leveraging standard IT virtualization technology to consolidate many network equipment types onto industry standard high-volume servers, switches and storage, which could be located in data centers, network nodes and in the end user premises,” the white​ paper stated.
Further defining the difference
Gaurav Jain of ETH Zurich compared network virtualization to server virtualization in that both practices begin with physical machines. In each case, IT workers then “carve out” the virtual machines inside these components to support a range of applications and processes. This approach results in optimal utilization of all the resources the hardware has to offer, Jain noted.
“[I]n network virtualization, you start with a single physical network (comprising nodes and switches) and ‘carve’ out multiple virtual networks, delegating the administrator of each virtual network to the same or different entity,” Jain stated.
While virtualization is an approach to providing support for the network in a virtual environment, SDN seeks to control how information packets are routed through these networks. In a typical network, routing and forwarding is carried out by the switch or router. However, with SDN in place, these needs are addressed by a central server.
“Whenever a router/switch gets a packet that it doesn’t know how to handle, it contacts the central server,” Jain stated. “The central server then either installs a ‘rule’ on the router/switch specifying how to handle packets of that particular flow or just replies with a port on which the packet should be forwarded.”
How SDN and NFV cross paths
Although these technologies can operate separately of one another, the current market environment shows that organizations are likely to implement the two side-by-side.
“[T]hese two revolutionary networking developments don’t represent an either-or proposition,” wrote CIO contributor Ed Tittel. “In fact, it instead looks very much like a both-and deal – as in ‘both SDN and NFV are likely to find a place in modern enterprise networks as carrier infrastructures.'”
When deployed together, SDN and NFV can help foster the creation of network comprised of more universal hardware components and open software. In this way, Tittel notes that SDN will carry out the control and management of the network and NFV will process the virtualized functions.
“It appears, then, that the kinds of functions that NFV seeks to deliver will work well within the framework designed for SDN,” Tittel wrote.