Over the last decade the growth of e-commerce, along with the extensive use of the Internet and intranets has pushed computer network security to the forefront. This seminars looks at a rather neglected but very important area, Protocol Analysis or sniffers as the units are commonly called.The goal of this seminars is to define packet sniffing, as well as to analyze the ways a packet sniffer eavesdrops. While analyzing the way a packet sniffers works, one has to understand what its original usage was, and how using a packet sniffer became eventually a threat. Moreover, this seminars?s goal is to describe not only how packet sniffing throughout the network can be achieved, but also the ways a packet sniffer can be detected. Another issue that will be considered is protection against packet sniffers.Finally, some popular sniffers will be addressed, including where they can be found. The seminars concludes that more can be done to combat security penetration using protocol analysis but fuller network security will require a combination of a large number of different tools and techniques.