I’m sure this is a very basic question.
I have a DDNS name that I was planning to register with Let’s encrypt (firstdomain.com). On my internal network I have a private DNS server with a non-registered domain (seconddomain.com) and I am running an internal web server off of seconddomain.com. Can I put a the seconddomain.com in the alternative names field when registering the certificate or does it only allow subdomains from the firstdomain (e.g. foo.firstdomain.com, bar.firstdomain.com).