192.168.21.1 is either a internal IP address or 192.168.21.1 is a private or reserved IP address with limited information on record.
Not every IP address has owner information associated with it. This is the case with your lookup of 192.168.21.1 here on IPAddress.com. 192.168.21.1 falls in to one of these categories. The categories include either a internal IP address, private IP address, or reserved IP address.
All of these types of IP addressess like 192.168.21.1 have limited information available because they exist in private networks or configurations. Routers and network modems for example might use 192.168.21.1 in the default configuration. You would type 192.168.21.1 directly in your web browser to login the admin interface.
Examples of router and modem brands that might use 192.168.21.1 in their configuration include Cisco Linksys, Netgear, TP-Link, ASUS, D-Link, Netcomm, Zyxel, Edimax, Belkin, Thomson, and hundreds more.
In other times 192.168.21.1 may fall in a reserved IP range whose scope could be set aside for use in software, subnets, and private networks. Private networks exist to keep local traffic contained to a home or business, and not exposed outside of these areas.
Examples of private or reserved use include broadcast messages, local communications, service providers, testing, and future applications.