From FAQ - Let's Encrypt :
What IP addresses does Let’s Encrypt use to validate my web server?
We don’t publish a list of IP addresses we use to validate, because they may change at any time. In the future we may validate from multiple IP addresses at once.
Additionally, the only validation method that directly uses 443/tcp (tls-sni) is being phased out.
The remaining validation methods (http, dns) use tcp/80 (but follows redirects to https://) and (no port), respectively.