Windows Server – Change a DHCP Scopes Subnet Mask

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KB ID 0001177 Dtd 17/04/16

Problem

You have probably tried to do this and found he option 'greyed out' because in the DHCP management console you can't change the subnet mask of a scope. 

I needed to change a clients DHCP scope because I was changing their subnet from /24 to /16. I could have simply deleted and recreated the scope, but they has a lot of custom settings for their phone system, and hundreds of reservations (which I know is unusual).

 

Solution

I wrote an article a long time ago about migrating DHCP scopes, this process uses the same method to take a backup of the scope, then you can edit the subnet mask, and 're-import' the scope again.

First take a backup of the scope, open an administrative command window, and run the following command;

netsh dhcp server \\”Server name” scope “scope subnet” dump>c\:dhcp.txt

i.e.
netsh dhcp server \\Server-DC01 scope 10.0.0.0 dump>c\:dhcp.txt

Then open the C:\dhcp.txt file, and locate the subnet mask;

windows scope change subnet mask

Edit it accordingly, and save the changes.

windows dhcp change mask

 

You can now DELETE the live scope, then reimport the one you just edited with the following command;

netsh dhcp server import c:\dhcp.txt all

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Author: PeteLong

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