What does Windows error code 9979 (DNS_ERROR_SUBNET_ALREADY_EXISTS) mean?

DNS_ERROR_SUBNET_ALREADY_EXISTS

DNS_ERROR_SUBNET_ALREADY_EXISTS means an IPv4 or IPv6 prefix is already present in the selected client-subnet definition.

What to verify for DNS_ERROR_SUBNET_ALREADY_EXISTS

• Check existing prefixes before appending one; use the same canonical network and prefix length.
• Do not add overlapping prefixes merely to increase priority: policy matching should be expressed by policy design and processing order.
• Review policies that use this client subnet after any intended change.

Get-DnsServerClientSubnet

Microsoft: Get-DnsServerClientSubnet

Microsoft: Set-DnsServerClientSubnet

Microsoft: DNS Policies overview

Where the result is returned

This result is Win32 system error 9979 (0x000026FB) from winerror.h. AllStat describes it as “The IP subnet that is being added, already exists in the client subnet.”. The code is useful only together with the API that failed, because multiple Windows components can reuse system-error values while imposing different retry and cleanup rules.

Diagnostic sequence

• Call GetLastError immediately after the failing API and save this result, the function name, all relevant flags, and the target path, handle, service, device, account, or policy object.
• Capture the component log that owns the dns / subnet / already / exists operation and retain the original numeric value before a framework converts it to an HRESULT or exception.
• compare preconditions with the API documentation and reproduce with a minimal request before changing system-wide configuration.

Retry this result only after the resource or state named in “The IP subnet that is being added, already exists in the client subnet.” has changed. For invalid parameters, unsupported formats, missing objects, policy restrictions, and access failures, correct the input or configuration instead of immediately repeating the same call.

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