Web servers have recently become the main source of information on the Internet. Every Web server uses a Web log to automatically record access of its users. Each Web-log entry represents a single user’s access to a Web resource (e.g., HTML document) and contains the client’s IP address, the timestamp, the URL address of the requested resource, and some additional information. An example log file is depicted in Figure 1. Each row contains the IP address of the requesting client, the timestamp of the request, the name of the method used with the URL of the resource, the return code issued by the server, and the size of the requested object.