The DHCP address pool on a Windows Server machine was being exhausted, so we wanted to increase the number of available addresses.

Original scope: 192.168.10.0 / 255.255.255.0

• provides the pool 192.168.10.1 – 192.168.10.255 (255 addresses)

Desired scope: 192.168.10.0 / 255.255.254.0

• provides the pool 192.168.10.1 – 192.168.11.255 (510 addresses)

The usual way requires you to delete the scope and re-enter it manually which can be tedious and time-consuming if you have a lot of exclusions and reservations.  Windows provides a way to do this programmatically:

1. Open a command window.
2. c:> netsh dhcp server \\”Server name” scope “scope subnet” dump>c:\dhcp.txt
   (In our case, it was netsh dhcp server \\Server scope 192.168.10.0 dump>c:\dhcp.txt)
3. Edit the created text file and change the net mask and save the file
   (we changed 255.255.255.0 to 255.255.254.0)
4. Delete the scope in the DHCP program on the server
5. Finally, in the command window,
   netsh exec c:\dhcp.txt

When we did this, a couple of scope options had to be re-entered: 046 and 051

Downtime on DHCP (i.e., no scope entry) was about 15 seconds.

Remember that, after doing this, any device that has a statically-assigned IP address instead of using DHCP should have its netmask changed or devices using an address outside the original scope won’t be able to see it.  DHCP clients will get the new netmask automatically at the next renewal.

This is a recent change that is activated silently after an Outlook update.  To undo, add this registry key:

HKCU\Software\Microsoft\Office\16.0\Outlook\Options

DWORD: DisableAutoCompleteUpdate
Value: 1