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 connect 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. The TCP/IP suite is the basis for today's Internet, yet it lacks even the most basic mechanisms of authentication. As usage of the Internet increases, its scarcity of built-in security becomes more and more of a problem. This paper describes serious attacks against Internet control and management protocols, with an emphasis on the ICMP protocol, as well as some of the well-known vulnerabilities of the inter-domain routing protocols.