IP range: 0.21.0.0 - 0.21.255.255 (0.21.0.0/16) , IPv4
This range is also known as "Class B" and it contains 65,536 (65k) IP addresses with the first address starting at 0.21.0.0 and the last address of this range ends at 0.21.255.255.
When using the CIDR range (Classes Inter Domain Routing) classification it is also typical to call this range 0.21.0.0/16. Since IPv4 addresses are represented using 32 bits, this way of writing describes the range as all the addresses where the first 16 bits are preserved from the address 0.21.0.0. That means that there are 16 bits which are constant, and 16 bits that are changing. Hence, 65k addresses.
This is an inner range inside the parent range 0.0.0.0/8 that belongs to IANA - Local Identification. Inner ranges are associated seperately to different private organizations, so if you're interested in finding out which organization controls this specific range, you can browse deeper into the IP addresses themselves where we have a WHOIS tool that can reveal this information.