Systems and methods for domain classification using the network request behavior of clients are provided. The network requests of a plurality of clients are analyzed to determine a domain corresponding to each request. This information can be used to associate a set of domains with each individual client. Because of the reciprocal nature of a network request, the information is also used to associate a set of clients with each individual domain. Within the plurality of domains associated with the plurality of clients, there may exist known domains having a classification and unknown domains having no classification. Based on the correlation of clients and domains from their respective associations, the system generates domain classification information for at least one of the unknown domains.