The source – developers.google.com
Google published a full list of IP-addresses of Googlebot.
Webmasters can use it to identify Googlebot by its IP address. This is an alternative way to check who visited your site. It's useful if you don't want to make a reverse DNS query using the source IP address.
To verify that it is Googlebot that has visited the site, you need to compare the IP address of the scanner with the list of IP addresses of Googlebot.
Google is constantly updating this list, so it's best to check the file daily.
