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.”