Abstract: IP address-user number estimates may be determined by accepting IP address usage information associating various IP addresses with various client identifiers, and generating an estimate of a number of users for at least one of the IP addresses using the IP address usage information. If the client identifiers include cookies, the estimate of a number of users for at least one of the IP address might be generated, in part, by determining a number of cookies for the at least one IP address for a given period of time, and multiplying the first ratio by a second, predetermined, ratio of a number of users per cookie for the given time period. If the client identifiers are user-agent parameters and/or browser parameters (e.g., screen setting information, time zone, history length, whether or not Java enabled, number of plug-ins, mime types, etc.