I already have this working, but I'm hoping someone can help me to understand why. One of the services my company provides to our clients is DNS hosting, including rDNS if they can get their IPs delegated to us.
In the past I've always set up our rDNS zones using a / instead of a - as I understood that was proper CIDR notation for classless subnets.
For example, if ATT delegated this block to us: 12.171.xxx.yyy/29
Then we would set up the zone on our DNS server like this: yyy/29.xxx.171.12.in-addr.arpa
For some reason this absolutely wouldn't work on the most recent delegation, so after banging my head on the wall for a while I replaced the slash with a dash and this started working:
We have dozens of other delegations from ATT that worked with the / so I have no idea why this one suddenly started needing a - instead.
If it matters, we're using SimpleDNS Plus 5.1.
Thanks for reading!