ISPs obtain public IP addresses from regional Internet registries (RIRs) which manage the distribution of IP addresses in their respective regions. RIRs ensure a fair distribution of the limited resource of IP addresses, and the adoption of IPv6 has expanded the available address space.
Internet Service Providers (ISPs) obtain public IP addresses from regional Internet registries (RIRs). These RIRs are responsible for allocating and managing IP addresses within specific regions around the world. The five RIRs are ARIN for North America, RIPE NCC for Europe, the Middle East, and Central Asia, APNIC for the Asia-Pacific region, LACNIC for Latin America and the Caribbean, and AFRINIC for Africa. ISPs apply to these registries for blocks of IP addresses, which they then assign to their end users. It's a hierarchical system designed to efficiently distribute the limited number of available IP addresses across the globe.
IP addresses are a finite resource, and their allocation is carefully managed to ensure fair distribution and avoid exhaustion. With the implementation of IPv6, there is an increased number of addresses available, significantly reducing the risk of running out anytime soon. This transition is an ongoing process in the networking community.