Over last few decades design of Network Intrusion Detection System (NIDS) has been a challenging problem faced by research community. Although various approaches have been proposed to design efficient NIDS the engineering challenges still remain. Detection of unknown network attacks is one such challenge. Growing convergence of networks aided by complexity and wide reach of networks is fueling emergence of new types of network attacks, which traditional NID systems are failing to detect. In this paper the authors present a NIDS design scheme that efficiently detects such new and unknown network attacks.