What does Windows error code 9977 (DNS_ERROR_CLIENT_SUBNET_ALREADY_EXISTS) mean?

Status / error code:		Category:

Constant name:
Source:		Operating System:
Description: A DNS client subnet with the supplied name already exists.

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DNS_ERROR_CLIENT_SUBNET_DOES_NOT_EXIST	DNS_ERROR_SUBNET_DOES_NOT_EXIST

DNS_ERROR_CLIENT_SUBNET_ALREADY_EXISTS

DNS_ERROR_CLIENT_SUBNET_ALREADY_EXISTS means the requested client-subnet name is already defined on the DNS server. A client subnet is a named group of IPv4 and IPv6 prefixes used by policies.

What to verify

Inspect the existing definition before changing it; it may be referenced by one or more policies.
Confirm whether the required prefix should be added to the existing logical subnet rather than creating another name.
Keep names semantically stable, such as by region, site, resolver tier, or other policy purpose.

Get-DnsServerClientSubnet

Microsoft: Get-DnsServerClientSubnet

Microsoft: Set-DnsServerClientSubnet

Microsoft: Add-DnsServerQueryResolutionPolicy

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