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Changing DHCP scope with a different subnet mask

Posted on 2016-10-18
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Last Modified: 2016-10-25
I’m planning to extend my DHCP scope and remove some of the users from my server Vlan5 to the user Vlan6-7.

Current User Vlan6 192.168.6.0/24 - Vlan5 192.168.7.0/24
Current Server Vlan5 192.168.5.0/24
Windows Server 2012 with all my scope options.
Using a Cisco 3750 Switches

User Vlan6-7 to a /23 address so I will have enough addresses for the users that are on my vlan5.
What will be the best option to do this without an IP address conflicts?
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Question by:fofanah78
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3 Comments
 
LVL 31

Expert Comment

by:masnrock
ID: 41849021
You need to be sure that you are selecting subnets that do not conflict. For example, if you use /23 for vlan6 without changing anything, vlan7 will conflict.

So you may want to either have a totally new address range or be ready to change vlan7 as well as vlan6.
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LVL 12

Expert Comment

by:Bryant Schaper
ID: 41849066
All they all DHCP clients on the user VLAN?

How may endpoints and switches are impacted?

If it is not so many you can change the DHCP scope, and shutdown/no shutdown the interfaces or reload the switch if possible.  The other option is shorten the DHCP renew time to something small, make the switch after the currect time has elapsed, and the clients will renew with the new IP.

Or lastly, these are users, I assume, make the change overnight and ask them to reboot.
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Accepted Solution

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Mal Osborne earned 2000 total points
ID: 41849338
You will also have  a problem with existing DHCP leases.

A DHCP lease has a time associated with it. By default, on a Windows server, this is set to 8 days. Clients will attempt to renew a lease after half that time, so 4 days. This means that a client PC that obtained a lease with the old subnet mask may retain it for most of the week.

Thus, you would be best off reducing your DHCP lease temporarily. If you drop it to a 10 mins  a week before  you make changes, then client PCs pick up the changes quickly. There are downsides to this, so once changes have been made, switch back to an 8 day lease.
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