A Mobile Ad hoc Network (MANET), also known as wireless ad hoc network, is a self-configuration and infrastructure-less network of mobile devices connected wirelessly in a continuous manner. They are vulnerable to various kinds of attacks due to their dynamic nature and lack of a central control point. One of the most several network layer routing attacks in Blackhole attack. Among those routing attacks, black hole throws a challenge in interrupting and degrading the performance of data forwarding operation by dropping the data packets that are transferred through it. To ensure secure MANET an Intrusion Detection System (IDS) is being required. So in this research, we proposed an anomaly based IDS to detect black hole attack in MANET in turn computations have been performed using Neighbors Selective Acknowledgement. In the proposed method, all the nodes receiving data packet send an acknowledgment to the node from which it received it. If the source node receives an acknowledgment from a destination within the threshold time, the path is found to be protected against black hole node and originator takes no action. Otherwise, the source starts verifying nodes. Source node unicast verification message to all the nodes whose details it has stored. Our simulation experiment proves that detects black hole nodes efficiently with less false positive rate of detection and false positive rate.