Understanding CIDR notation
The "/24" suffix says how many bits identify the network: the bigger it is, the smaller the subnet. A /24 gives 254 usable addresses, a /30 only 2 — handy for point-to-point links.
Reserved ranges
Some ranges never travel the public Internet: 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16 (private networks), 127.0.0.0/8 (loopback), 100.64.0.0/10 (carrier CGNAT). The tool flags the type of the range you enter.