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Add subnet to network

Posted on 2014-11-06
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Last Modified: 2014-11-10
So i need to expand the main network for a client. They currently are on a 10.10.10.0/24 scheme. If i'm correct, the process would involve changing them to a /23, which would open up the 10.10.11.0 subnet to be used by DHCP. I know its not that simple, so i wanted to review what needs to be done in order to accomplish this. Currently everything is setup with subnet masks of 255.255.255.0 which will need to be changed to .254 for anything with a static IP address? The gateway is currently 10.10.10.1, so i should not need to modify anything on that end, and then add an additional DHCP scope to include the 10.10.11.X addresses.

I know i'm overlooking steps, so feel free to list/ask me questions.

As always, i appreciate the help.
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Question by:altitudetek
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6 Comments
 
LVL 23

Accepted Solution

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Thomas Grassi earned 250 total points
ID: 40427144
If most of your devices are DHCP clients then drop the lease time down to less than an hour before your ready to change.

Change all the static ip addresses you have defined. Servers Printers etc.

your new network will be

10.10.10.0
subnet mask 255.255.254.0
mask bits 23
number of subnet 32768
number of hosts 510
network address 10.10.10.0
first usable address 10.10.10.1
last usable address 10.10.10.254
broadcast address 10.10.11.255

Create a spreadsheet with all the know ip address ranges and static and reservations you may have then you will have a good check off list.
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LVL 35

Expert Comment

by:Seth Simmons
ID: 40427175
if it's /23 then the last usable address would be 10.10.11.254
10.10.10.254 would be /24

shouldn't have to create new dhcp scope for the .11 range; the .10 should cover it when changing to /23
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Author Comment

by:altitudetek
ID: 40427182
Both good to know, in terms of the static addresses are there anything i need to concern myself with regarding gateway and subnet. As far as i could think changing the subnet mask and leaving the gateway (since it remains unchanged) was all that would require. Nothing would change on the firewall aside from the same change to a /23 network
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LVL 23

Expert Comment

by:Thomas Grassi
ID: 40427197
Only change will be the subnet mask.

As seth says you do not need to create a new scope

Just modify the existing scope when your ready.
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LVL 46

Assisted Solution

by:Craig Beck
Craig Beck earned 250 total points
ID: 40429849
Be careful with this.  It's straight-forward but you will be making a couple of IP addresses usable which would normally be a broadcast and network address - 10.10.10.255 and 10.10.11.0.

Because you're changing the mask to /23 these addresses will be valid host addresses, BUT some devices won't like using them if they are assigned via DHCP as some clients don't work out the broadcast address correctly.  To mitigate this I would exclude those addresses from the scope.

Also, if you're using a Microsoft DHCP server you will have to delete the scope and recreate it as you can't adjust the subnet mask once the scope is created.  If you have reservations and exclusions that you want to keep you should export the scope using NETSH then edit the output to include the new subnet mask and import the scope again using NETSH...

netsh dhcp server \\YOURDHCPSERVER scope 10.10.10.0 dump >c:\dhcpscope.txt

Open in new window


Edit the subnet mask for the scope in the c:\dhcpscope.txt file then save.

Delete the scope from DHCP then...

netsh exec c:\dhcpscope.txt

Open in new window

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Author Closing Comment

by:altitudetek
ID: 40432842
Thanks for the input. I'll have to create a good spreadsheet for all the static's on the network but it looks like i have what i need to move forward with this.
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