Mobile Ad Hoc Networks (MANET) are typically wireless networks that do not have any fixed network architecture. This makes the task of routing layers difficult. A popular reactive routing protocol – Ad hoc On-demand Distance Vector (AODV) used in MANETs has certain vulnerabilities which make it susceptible to black hole attacks. This paper proposes an innovative technique to detect and prevent the possibility of a black hole attack in AODV based MANETs. Using the approach of detecting multiple Route Reply (RREP) messages, authentication of nodes is done by the use of a randomly generated id unique to each route discovery process. This method was tested using simulations in NS2 software and compared with previous attempts. The proposed method showed marked improvement in performance in the presence of malicious nodes.