and 2001:db8:0:1234:0:567:8:1 in IPv6. The size of the routing prefix of the address is designated in CIDR notation by suffixing the address with the number of significant bits, e.g., 192.168.1.15/24, which is equivalent to the historically used subnet mask 255.255.255.0...
If you're not sure what to enter here, just refer to the CIDR cheat sheet at the bottom of the tool. For example, if you're working with a range of 256 IP addresses (a common subnet size for homes and small offices), you'll use /24. For /24 ranges, the subnet mask will be...