As computer networks continue to proliferate, the world's dependence on a secure communication infrastructure is of prime importance. Disruption of service through Denial of Service (DoS) attacks can result in great financial loss for Internet-based companies and major inconveniences for users of Internet services. The purpose of this two-year study was to study and understand network denial of service attacks so that methods may be developed to detect and prevent them.;Initially, the researcher constructed a database of system and network exploits that revealed the underlying vulnerabilities in the software or protocols they attack. The database was populated with exploits posted at popular reporting sites such as Rootshell, Bugtraq, Security Focus. To encourage the use of a common vulnerability taxonomy and to facilitate sharing of data, parts of the classification scheme proposed by Krsul (1998) in his research were included, as well as developing a taxonomy tree based on the current research.;Sifting through the reports and categorizing the attacks has been a challenging experience; and creating categories that are unambiguous, repeatable, and exhaustive has proven to be a difficult task. The results were two to three methods of classification that are useful for developing categories of vulnerabilities. The next phase of the project was to look for any clustering of attacks based on these vulnerability categories, and to determine if effective countermeasures can be deployed against them. Although past history is no guarantee of future exploit activity, it is hoped that the countermeasures proposed based on these 630 exploits will remain valid for future DoS attacks. Toward this goal, the research made use of data mining software packages to plot the various categories of attacks so that the interrelationships could be more easily discovered and studied. A sampling of the database plots, an interpretation of the plotted data, and the countermeasures proposed for the vulnerability categories developed as part of the database creation are presented in this research.