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Change subnet mask to get more addresses

Posted on 2014-10-08
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Last Modified: 2014-10-12
I have a network 192.168.111.0 /24 and I am running out of Dhcp addresses.  I do not want to add another Vlan even though I know that is an option.   I want to change the mask to /22 255.255.252.0 to gain the addresses that way.  My plan is to change the mask on the firewall LAN interface.  Create a new Dhcp scope on the firewall using the new range.  Dhcp devices will need to renew to get a new address.  For the static devices I need to change their mask manually.  My question is will I be able to connect to the static devices remotely when they have the old mask if I've made the change on the firewall already?  I am trying to do this without going to the site and want to verify it will work.  

Old range is 192.168.111.0 - 255

New range 192.168.108.0 - 192.168.111.255
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Question by:dmwynne
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by:omarfarid
omarfarid earned 167 total points
ID: 40369066
I would suggest to connect to each device / systen and change its subnet mask before doing it in firewall or dhcp
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by:David Johnson, CD, MVP
David Johnson, CD, MVP earned 166 total points
ID: 40369220
as long as the new netmask is less restrictive than the old netmask you will be able to connect without any issues.  In other words as long as the network address fits in the defined network area defined by your netmask.
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Fred Marshall earned 167 total points
ID: 40369970
Mostly yes.  The issue is whether you need the broadcast addresses to match and, WHEN.
In most cases, it won't matter.
I always change the gateway and DHCP *first*.
Newly addressed devices will be in line with the new settings.
Old settings will still work because:
- If packets are addressed within the old subnet then they will just go out on the wire and that's all there is to it.
- If packets are addressed to the expanded subnet (why would they be?) then they will be directed to the gateway; the gateway will put them back out on the wire and they will reach their destinations after that one hop.
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by:pergr
ID: 40370373
In order to avoid disruption, you need to keep the firewall IP (which is the default gateway in DHCP and for the hosts) within the 192.168.111.x range.
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by:vivigatt
ID: 40370379
It will work provided that you do not have any node that currently has an IP address in the "new pool" of addresses .
You must:
- keep all existing routing devices DNS, DHCP, WINS etc. with their existing IP addresses, just change their subnet mask
- change subnet mask on the static devices but keep their existing IP addresses and router/DNS settings
- delete the existing dhcp scope and create a new one with the new subnet mask. Remember to exclude the static devices from the pool.

Some resources that will help you:

http://www.subnet-calculator.com/
http://support2.microsoft.com/kb/255999
http://technet.microsoft.com/en-us/library/dd145324%28v=ws.10%29.aspx
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