Recommendation ITU-T Y.3320 describes overview and requirements for applying formal methods to SDN. In Appendix, it introduces an example to show how to apply formal methods to SDN environments. Formal methods are mathematics-based techniques to specify, develop, and verify software and hardware systems, and are expected to increase reliability and robustness of the system. In SDN environments, consistency, reliability and safety of applications are important since incomplete or malicious programmable entity could cause break-down of underlying networks. In this sense, the usage of formal methods can be an effective approach to mitigate the problems.