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Why Consider this
Subnets are used to allow clients and servers to find the nearest Active Directory domain controller. Without the correct configuration, for example delays in authentication can occur.
Watch a Customer Engineer explaining the issue
Context & Best Practices
The subnet definitions listed in this issue are derived by the IP addresses of clients authenticating from undefined subnets. A class C, or 255.255.255.0, subnet mask is assumed. This may not be accurate if the environment uses other subnet masks. The list is meant to be a starting point to assist environments in defining all subnets properly in Active Directory. Refer to the issue titled "Missing Subnet Definitions" for details on how undefined subnets impact Active Directory.
Suggested Actions
Review the list and add the missing subnets to the appropriate site if possible.
How to Troubleshoot
Determine if these subnets are required.
Work with your networking team to identify the closest site for each subnet.
Create subnets in the AD Topology and link them to the correct site.
Learn More
Using Catch-All Subnets in Active Directory: https://technet.microsoft.com/magazine/2009.06.subnets.aspx