Say I have domain.com and its hosted externally.
I add an exchange server and, I add an external record pointing to my server called mail.domain.com and it points to my external IP.
I ALSO want to be able to access my server using the internal IP instead of going through my firewall and back in. (This is called split DNS)
Split DNS = I have 2 DNS zones, one external and one internal for the same domain.
The issue is that you have to manage both zones individually (even if you only need one specific host record)
And alternative method is to create a zone JUST for that one host name.
Here are the directions to create a domain and same as parent A record
Now you have split DNS for the single host name only.