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DHCP Question

Posted on 2014-07-23
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Last Modified: 2014-07-28
Simple question ...

My DHCP server is running Windows 2003 R2. The subnet mask is 255.255.254.0, effectively giving me 510 IP addresses.

I am running out of IP addresses.

Can I change the subnet mask to 255.255.252.0, giving me 1022 IP addresses -WITHOUT- losing all of my reservations?

This is a simple, flat network here.

Thanks.
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Question by:glenj1978
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LVL 7

Expert Comment

by:CorinTack
ID: 40215329
The closest you can get is exporting your reservations first, creating the new range, then importing them.

See the following article (and the first few comments) for how to accomplish this (the comments are necessary to understand what you need to do to get the reservations to import with the altered IP scope): http://koolbeans.wordpress.com/2007/07/31/howto-import-and-export-dhcp-reservations-in-server-2003/
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LVL 9

Expert Comment

by:Sandeep Gupta
ID: 40216056
since you are changing the subnet mask thus you have to flush all the existing with .254.

This is very simple change of not more than 5 minutes..with minimum impact
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vivigatt earned 2000 total points
ID: 40216468
Check this article:
http://support.microsoft.com/kb/255999

It does not mention how to keep your reservations, but check this article:
http://www.mpking.com/2013/08/using-netsh-to-export-and-import-dhcp.html

So now, a path is clear:
Use the netsh "dump" method described in the 2nd article to export your reservations

Extend your DHCP scope by resubnetting (use http://subnet-calculator.com if you need help with subnet ranges, starting and ending addresses etc) as described in the 1st artcile.

Import the DHCP reservations as described in the 2nd article.

Voilà !
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Author Comment

by:glenj1978
ID: 40222826
Sandeep ...

I realize its a 2 minute change to change the subnet mask and instantly have 1022 IP addresses to give out, but I don't want to lose my 125 address reservations and have to re-enter them manually.

With all the millions of little utilities that have been written by everyone and their mother over the last 30 years, you'd think SOMEBODY would have written one that exports the reservations, changes the subnet mask, and imports the reservations back. Ugh :)
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LVL 16

Expert Comment

by:vivigatt
ID: 40224687
The 2nd article I mention describes exactly how to export and import a list of DHCP reservations.
You will need to edit some text file but it should not be too complex.
If you can't figure it out, just run the command:
netsh dhcp server dump > dhcpcfg.txt

Open in new window

and attach the dhcpcfg.txt file to your reply, I'll create the modified text file for you and I'll let you know how to use it...
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