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Change subnet mask to get more addresses

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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dmwynne
Asked:
dmwynne
3 Solutions
 
omarfaridCommented:
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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David Johnson, CD, MVPOwnerCommented:
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 MarshallPrincipalCommented:
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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pergrCommented:
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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vivigattCommented:
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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