Hi @maikell, as @jsha said, the Let's Encrypt CA does not want to announce particular IP addresses that are used in validation because of a desire to change them periodically (partly in order to make it harder for attackers to be able to cause misissuance). While you could figure out what addresses are currently used, they may change at any time and will not be documented. If you can't allow inbound connections from the general public to the service that you're trying to validate, you can use the DNS challenge type (which just requires letting the Let's Encrypt CA look up your DNS records associated with that name).