MANET stands for "Mobile Ad Hoc Network." Mobile ad hoc network (MANET) is an autonomous system of mobile nodes connected by wireless links. Each node operates not only as an end system, but also as a router to forward packets. The nodes are free to move about and organize themselves into a network. These nodes change position frequently. A MANET is a type of ad hoc network that can change locations and configure itself on the fly. Because MANETS are mobile, they use wireless connections to

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... ect to various networks To accommodate the changing topology special routing algorithms are needed. There is no single protocol that fits all networks perfectly. The protocols have to be chosen according to network characteristics, such as density, size and the mobility of the nodes.There is still ongoing research on mobile ad hoc networks and the research may lead to even better protocols and will probably face new challenges. Current goal of this paper is to find out the security Issues and their Countermeasures that are adopted on the Network Layer. Network security extends computer security, thus all the things in computer security are still valid, but there are other things to consider as well. Computer security is defined as follows: -Broadly speaking, security is keeping anyone from doing things you do not want them to do to, with, or from your computers or any peripherals In MANET, the nodes also function as routers that discover and maintain routes to other nodes in the network. Establishing an optimal and efficient route between the communicating parties is the primary concern of the routing protocols of MANET. Any attack in routing phase may disrupt the overall communication and the entire network can be paralyzed. Thus, security in network layer plays an important role in the security of the whole network. A number of attacks in network layer have been identified and studied in security research. An attacker can absorb network traffic, inject themselves into the path between the source and destination and thus control the network traffic flow. Procedia Computer Science 3 (2011) 954-960 www.elsevier.com/locate/procedia 1877-0509 c ⃝