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.
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: