There is a finite number of IPv4 addresses, you can simply open TCP connection to each of them on multiple ports and index everything that is returned. There ar

There is a finite number of IPv4 addresses, you can simply open TCP connection to each of them on multiple ports and index everything that is returned. There are tools to do this by yourself (the only challenge is to find a provider that won't ban you for scanning the entire internet). There are also other services that do the same thing (e.g. ZoomEye, Censys, FOFA) For IPv6 there are too many addresses to scan them all so obtaining an IP to scan is a slightly different story (e.g. https://arstechnica.com/information-technology/2016/02/using-ipv6-with-linux-youve-likely-been-visited-by-shodan-and-other-scanners/ https://arstechnica.com/information-technology/2016/02/using...) but once you get the address the scanning process looks the same.