Network survivability is the research field and engineering concept focused on the capacity of a network to continue providing services and maintaining functionality despite failures, attacks, or large-scale disruptions. It investigates the design principles, architectural strategies (such as redundancy, distributed control, and dynamic routing), protection/restoration mechanisms, and quantitative metrics required to ensure network resilience and high availability against various sources of interruption, ranging from component failures to catastrophic events, thereby ensuring reliable communication and service continuity.