For every organization having a well secured network is the primary requirement to reach their goals. A network is said to be secure if it can protect itself from sophisticated attacks. Due to the rapid increase in the number of network users, security becomes the main challenge in the area of network field. Most security related threats target the layers of the OSI reference model. Sophisticated attack types such as Distributed Denial of Service (DDoS), Man-in-the-Middle and IP spoofing attacks are used to attack these layers. In this paper, we analyze most of the attack types that cause serious problems in computer networks and defense techniques to stop or prevent these attacks.