what is cidr?
the notation for writing a block of ip addresses as a starting address plus a prefix length, like 203.0.113.0/24.
classless inter domain routing replaced the old class a, b and c system in the nineties. the number after the slash says how many leading bits are fixed; the remaining bits vary across the block. a /24 fixes 24 of 32 bits and covers 256 addresses, a /16 covers 65,536, a /32 is a single address. ipv6 uses the same syntax over 128 bits, where a /64 is one ordinary network and a /48 a typical customer allocation. as a buyer you meet cidr in three places: providers describe their address space in blocks, public blocklists are published in cidr, and firewalls and allowlists accept it. it also explains why blocking is so coarse. a site that sees abuse from one address will often block the entire /24 around it, taking 255 uninvolved neighbours with it, which is why the block an ip sits in tells you as much as the ip does.