How the generator works
Two modes, one form. Which one you get depends on the Base IP field.
Leave Base IP empty and you are in random mode: we pull the requested number of addresses from a cryptographically secure random source. They belong to no device you know about, so they are safe for test environments, mock data and documentation.
Type a Base IP and you are in range mode: we apply the prefix mask, find the network address and list the first N addresses from there, in order. Enter 192.168.1.50 with a /24 prefix and the list starts at 192.168.1.0. Handy when you plan allocations or test firewall rules against one subnet.
Tick Public IPs only and every RFC 1918 private range (10.x.x.x, 172.16-31.x.x, 192.168.x.x), loopback, link-local and the other reserved blocks are skipped, so each result is a routable public address.
Where people use it
Four jobs a list of made-up addresses does well.
Application testing
Fill a test database with addresses that look right without touching real customer data. You never hit live infrastructure by accident, and the test set is anonymised by design.
Network simulation
List a whole subnet and run it against routing configs, firewall rule sets or a load balancer in the lab, where real addresses are not available.
Security research
Feed synthetic address sets to the scanner, parser or log script you are writing, so it handles the full IPv4 range without sending a single real packet.
Documentation and presentations
Swap the real addresses in screenshots and network diagrams for generated ones and stop leaking your internal topology.
How much IPv4 space there is
The numbers behind the 32 bits.
IPv4 addresses are 32 bits long, so there are 4,294,967,296 (2^32) of them in theory. Roughly 600 million are reserved for private networks, loopback, multicast, documentation and other special uses, which leaves about 3.7 billion routable public addresses. IANA handed the last unassigned /8 blocks to the Regional Internet Registries in February 2011, and the central pool has been empty since.
IPv6 is the way out, but IPv4 still carries most traffic. NAT keeps it alive by letting many devices share one public address, at the cost of complexity and broken end-to-end connectivity. Subnetting IPv4 is still a skill every network admin and developer needs.