Public IP Address assignment model

Step. 1 ICANN and IANA group public IPv4 and IPv6 addresses by major geographic region.
Step. 2 IANA allocates those address ranges to Regional Internet Registries (RIR)
Setp. 3 Each RIR further subdivides the address space by allocating public address ranges to National Internet Registries (NIR) or Local Internet Registries (ISPs are typically LIRs)
Step. 4 Eacht type of Internet Registry (IR) can assign further subdivided range of addresses to the end user organization to use.