Abstract: A Mobile Adhoc Network (manet) is a cooperative engagement of a collection of mobile nodes without any centralized access point. The underlying concept of coordination among nodes in a cooperative manet has induced in them a vulnerability to attacks due to issues like dynamically changing network topology, cooperative algorithms and lack of centralized monitoring point. We propose a semi-distributed approach towards a reputation-based Intrusion Detection System (ids) that combines with the Dynamic Source Routing (dsr) protocol for strengthening the defense of a manet. Our system inherits the features of reputation from human behavior, hence making the ids socially inspired. It has a semi-distributed architecture as the critical observations of the system are neither spread globally nor restricted locally. The system assigns maximum priority to self observation by nodes for updating any reputation parameters, thus avoiding the need of a trust relationship between nodes. Our sy...