How to determine the IP of a site?

Question

Domain Name System

There are services that determine the ip of the site, for example 2ip.ru Questions:
1. How does it work? IP is determined by means of php or do they refer to some source that shows the ip of the site? Whois, for example, does not seem to show ip
2. How are other sites determined on this ip? Again, your own base for comparison or the original source?

Answer 1

ping yandex.ru
Definition in PHP:
php.net/manual/ru/function.gethostbyname.php
On sites, this works by creating a database with ip addresses and domain names.