This becomes even more apparent when there are multiple CUCM clusters as they can share the same corporate directory.
In the case of IPv6 reverse-mapping was originally mandatory but as part of the seemingly relentless move to relax the goals of the original specifications (doubtless for good operational reasons) it is no longer a mandatory requirement.
The LDAP User Search Base uses two attributes to make up a dn (distinguished name).
This includes the cn (common name) and the dc (domain component).
Historically IPv4 address were allcated in /8 blocks to RIRs but due to IPv4 address depletion this is no longer possible.
IPv6 addresses are typically allocated to the RIRs in /12 to /23 blocks depending on demand and cicumstances.