Building blocks for software-defined networks (SDN) require which of the following?

While SDN controllers and network functions can run as virtual machines, SDN is not defined by or 'mostly composed of' VMs - its defining characteristic is the separation of control and data planes via a client-server communication model, not virtualization of hardware.

SDN architecture is fundamentally built on client-server pairs: the SDN controller acts as the server that centralizes control-plane logic, while network devices (switches, routers) act as clients that receive forwarding instructions via southbound APIs (e.g., OpenFlow). This separation of control and data planes is the defining building block of SDN, enabling programmatic network management and automation.

Virtual memory is an OS-level memory management technique unrelated to SDN architecture; SDN building blocks are defined by control/data plane separation and programmability, not by memory type preferences.

RAM versus virtual memory is an operating system or hardware performance consideration and has no bearing on the architectural building blocks or defining characteristics of software-defined networking.