My MS guy isn’t here but from my master’s hosts file I have comments that say:
Exchange DAG backup starts with DAG host name (DAG1, DAG2, DAG3 etc...) but
then dispatches to preferred node to do the actual data backup. It switches to other node if preferred isn't available.
So the DAG address (DAG1, DAG2, DAG3) is the 10.x non-private LAN but for the last DAG we added (DAG 3)
I created private LAN addresses in the 172.x range like the rest of our backup LAN IPs and it worked.
So if you had a 2 node DAG setup you might have entries for
DAG that had a single IP for that DAG in the non-private network.
host1 that has a private network IP for the first (preferred) node.
host2 that has a private network IP for the second (preferred) node.
Not sure how that is configured on the MS side though.