CIDR: Classless Inter-Domain Routing

The number after the slash says how many leading bits are fixed. A /24 holds 256 addresses, a /16 holds 65,536, and a smaller number means a larger block. IPv6 uses the same notation with longer prefixes.

Lists of cloud, proxy and abuse addresses are published as CIDR blocks, so checking whether an address is covered is a prefix match.

You can turn a prefix into a range by counting host bits. An IPv4 address has 32 bits, so a /24 leaves 8 host bits and 2^8 = 256 addresses. Published lists often hold overlapping blocks, so a lookup should return the most specific match, which is the longest prefix, and not just the first one it finds.