Certain embodiments involve associating a device with an identifier of a router that is assigned an IP address based on DHCP. For example, connection data that includes the IP address is analyzed to associate devices with the identifier of the router. A determination is made as to whether any usage of the IP address by the devices overlap in time. Devices with overlapping usage are determined to be connecting to online resources via a same router, while devices with non-overlapping usage are determined to be connecting from other routers. A single router identifier is associated with the devices using the same router. This identifier is then used to track the online activity of the associated devices.