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On Server 2008 R2 - Expand /24 to /16 Netmask

Posted on 2016-09-09
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Last Modified: 2016-09-13
Client has small network, 1 AD/DHCP/DNS Box - Server 2008 R2 Enterprise with one 192.168.1.x/24 network. Has been expanding throughout years with computers, ip phones and ip cameras and have run out of ip addresses. What is the easiest way to acquire more IPs? I assume by changing subnet mask to /16? Is there a way to do this with very little downtime to network? Can I and if so, how, export my current dhcp scope and reservations, modify the file to /16 then import back? Thanks for help.
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Question by:xav1963
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RantCan earned 500 total points
ID: 41791485
Broad steps:
1. Change DHCP Lease time to 15 minutes;
2. Create new scope with changed Netmask (recommend going to 192.168.1.x /22 which gives you 192.168.0.x, 1.x, 2.x, 3.x  as well)
3. Activate new scope
4. Deactivate old scope
windows clients should renew with new scope.

Caveats:If you are not using VLANS, you must be cautious because this is still a single collision domain, and you might increase your broadcast traffic an intolerable amount, based on your description.  I recommend setting a VLAN for IP phones (so you may apply QoS), as well as one for servers, one for cameras, reserving  a single VLAN for workstation traffic.
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by:xav1963
ID: 41791515
understand ... so there is no need to create a superscope?
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by:RantCan
ID: 41791542
Not if you are moving to VLANs, which is recommended. I would try to keep the individual VLANs as flat as possible; superscopes don't add any advantage if you have a properly VLAN segmented routing network. Is your infrastructure capable of properly routing VLAN traffic. If the answer is yes, no superscopes. This article explains in technical details why superscopes are generally useless in a properly switched network.  http://robertparten.com/dhcp-multiple-scopes-vs-superscopes/
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Author Comment

by:xav1963
ID: 41791857
understand ... just need a few more IPs ... will investigate network and tackle vlans later ...
tried your instructions and getting error due to overlapping network, I would have to delete old scope first, losing existing leases or reservations if possible ...
I tried this link but also am getting errors ... https://www.petenetlive.com/KB/Article/0001177

Still back at square one ... need to create new scope of 192.168.1.0/22 and save existing leases and reservations at same time ....
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by:RantCan
ID: 41791922
DHCP protocol will attempt to renew it's lease halfway through it's current lease. With this in mind, if you set your lease to 4 hours, you'll have two hours to make the changes. If you do it on the weekend, and test out, then you have enough time to perform the functions.  The linked article seems straightforward.  What errors are you getting using the dump/edit/delete/import steps?
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Expert Comment

by:Qlemo
ID: 41792867
RantCan, changing the DHCP lease time does not help if machines do not get restarted or forced to renew DHCP leases. If machines are running 24/7, they will only ask after half of the original lease time for rconfiirmation of their IP.
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by:RantCan
ID: 41792881
Which is why I recommended to SHORTEN the lease time prior to undertaking this config change.
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by:Qlemo
ID: 41792886
Imagine you have a lease time of 14 days - which is not uncommon.Then you cannot get this work before a week has passed ;-).
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Author Closing Comment

by:xav1963
ID: 41796567
thanks for answering question ... think I asked question incorrectly ... will try again later..
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