Attempting to add an Exchange 2010 server to a DAG throws the error: “WARNING: Network name 'DAG-01' is not online. Please check that the IP address configuration for the database availability group is correct.”

Problem

You attempt to add a newly built Exchange 2010 mailbox server to an existing DAG but receive the following error:

[PS] C:\Windows\system32>Add-DatabaseAvailabilityGroupServer -Identity DAG-01 -MailboxServer MBX-01
WARNING: Network name 'DAG-01' is not online. Please check that the IP address configuration for the database
availability group is correct.
[PS] C:\Windows\system32>

Solution

While there are probably many different reasons why this error would be thrown, the issue I had was because I had an incorrect subnet mask.  The environment I was working in had a /16 subnet for their server subnet (don’t ask me why) and I had incorrectly used a /24:

Once I corrected the subnet mask to a /16:

The Add-DatabaseAvailabilityGroupServer cmdlet ran successfully and the new mailbox database was added to the DAG: