Abstract: A Mobile ad hoc network (MANET) is a kind of network where the mobile devices are with a dynamic structure. Here each node in the network participates in routing by forwarding data packets to other nodes. Security becomes a crucial concern to ensure protected communication between nodes in ad hoc networks. There are several of challenges with respect to security design as ad hoc network is a decentralized and infrastructure less type of wireless network. There are five layers in MANET and each of these layers is prone to various kinds of attacks. This paper discusses about two important attacks in network layer, their defence mechanisms, those are: Cooperative Black Hole Attack and Rushing Attack.Mobile ad hoc networks (MANETs) are mostly and widely used in military and civilian applications. Due to the dynamic topology of MANETs, it allows the nodes to join and leave the network at any point of time. This generic characteristic of MANET is a cause which makes it vulnerable to security attacks. In this paper, I address the problem of coordinated attack by multiple black holes acting in group and rushing attack. I introduce a technique to identify multiple black hole nodes cooperating with each other and rushing nodes and solutions to discover a safe route avoiding cooperative black hole attack and rushing attack.

Keywords: MANET, Destination Sequenced Distance Vector routing (DSDV), Dynamic Source Routing (DSR).