... is not always straight forward. This is especially so if you are running in a NAT environment and want to deploy IPv6 and do host resolution correctly. Internally, you want to resolve a host as:
Since this node makes connections to the world, we need to resolve it's address for some services to work:
Before IPv6 was enabled on this network, this node would be resolved via a generic NAT IP address.
Now you have an asymmetry in how access to/from test1.example.com occurs which means it can work for some people (internal hosts, IPv6 enabled remote hosts) but not for others (IPv4-only sites.) In general, asymmetry in your security with two different paths to the same host means you are less secure. Unfortunately, IPv6 is more than IPv4:IPv4:IPv4:IPv4 and requires some thought and expertise that many shops just don't have.