One of them:
http://community.nethserver.org/t/replace-dnsmasq-with-bind9/2657
https://www.domainmonster.com/glossary/
https://www.123-reg.co.uk/support/answers/Domains-Archive/Domain-Configuration/what-is-an-a-record-257/
https://support.dnsimple.com/articles/a-record/
http://dns-record-viewer.online-domain-tools.com/
https://support.google.com/a/answer/48090?hl=en#H
…
Of course, but you can reach to your internal systems (www, mail, …) with Port forward or with Reverse Proxy (publish them to Internet).
Also, maybe I’m wrong! I can admit this! 