Mobile Ad Hoc Network (MANETs) is a network of mobile nodes which can move freely. These nodes have characteristic that they can be dynamically self-organized into arbitrary topology networks without a fixed infrastructure. MANETs are highly dynamic network because nodes may join and leave the network at any time. The Ad hoc on demand distance vector routing is a reactive routing protocol that finds the route on demand i.e. when one node wants to send message to another node. The different network layer attacks such as worm hole attack, black hole attack, packet dropping and message tampering attack causes network operation disturbance. Security at network layer can be provided using different approaches. These approaches used to detect and prevent such attacks in MANET. In this paper we review some techniques that have prevented such attacks.
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