Hi,
We've had a related thread here where it was explained that there is no specific IP address that can be used consistently.
It would be easy to find out which IP address is currently used, but there are plans in the work to use varying and potentially unpredictable IP addresses in the future so we've strongly discouraged somehow hard-coding the IP address. As @jsha said in the other thread, if you can't allow arbitrary inbound connections for the validation, the DNS challenge might be more appropriate for you.