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DHCP range has reached its limit and need to configure a bigger subnet

Posted on 2014-04-09
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Last Modified: 2014-04-16
To whom it may concern

I am the network administrator for a company and have a problem with the amount of ip addresses available.  

My configuration is as follows:
Server name: Server
Windows 2008 R2 Server Standard
This is also the only DHCP server on the network
The server is also the main DNS server
DHCP range = 192.168.0.50/24 - 192.168.0.80/24
Default Gateway = 192.168.0.200/24
The rest of the addresses are made up of static addresses with can be PC's Access Points, managed switches, payroll clocking systems and so forth.

Basically I would like to keep my Windows 2008 Server as the main DHCP server which also handles the DNS for the network and is also the host for their packhouse application.

2x main ip addresses that I want to keep = 192.168.0.20 (Windows 2008 R2 Server), 192.168.0.250 (Linux PC for Vitrax application)

I have read on the internet about subnetting and superscoping, but want to follow the best solution route to configure a subnet for the network that will allow me to use more ip addresses that 254.  

How do I go about to configure the DHCP Server to allow say for instance 4094 addresses and what will the subnet mask be that I should use?
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Question by:Lourensvd
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Accepted Solution

by:
David Johnson, CD, MVP earned 250 total points
ID: 39988376
You could change the netmask to 255.255.0.0 this will give you 65535 addresses
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Author Comment

by:Lourensvd
ID: 39988409
Hello David Johnson, CD

Thanks for the reply. I just want to make sure.  I can still use 192.168.0.x as my network address, but change the subnet from 255.255.255.0 to 255.255.0.0.  So this I can do on all my PC's and servers that have static ip addresses assigned to them?  Now to configure my DHCP Role on Windows 2008 R2 Server, do I delete and re-create it or can I just edit the DHCP Server already set up.  Will this also be the least disruption to the network?
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Expert Comment

by:Zephyr ICT
ID: 39988422
Yes you can keep using the existing addresses, just make sure you change the subnet on all network devices (routers, switches, printers, pc/workstations, servers, etc... ) ...

As for the DHCP, the easiest and quickest solution is to backup the existing config with netsh, change the subnet in the created file and delete the existing DHCP scope... Then use the adjusted file to recreate the scope with the correct subnet.

Something like this would do it I think:

netsh dhcp server \\”Your-Server” scope “ip-of-scope” dump>c:\dhcpbackup.txt

Then adjust the subnet in dhcpbackup.txt to the new subnet and add "add", something like this:

Dhcp Server \\server-name add scope 192.168.0.0 255.255.0.0 "Pool-Name" ""

Delete existing scope ...

With the adjusted file do this: netsh exec c:\newdhcpscope.txt
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Author Comment

by:Lourensvd
ID: 39988692
I just wanted to test the configuration in a virtual lab environment, before making the changes on the live server.  So I've setup a Windows 2008 R2 Server with the DHCP Role installed.  My configuration is as follows.
DHCP Server = 192.168.0.20
Default Gateway  = 192.168.0.200
DNS Server = 192.168.0.20

DHCP Range = 192.168.1.50 to 192.168.2.50
Length = 16
Subnet Mask = 255.255.0.0
Router = 192.168.0.200

*I've chosen this configuration because I want to test what will happen if all IP addresses to 192.168.0.254 have been taken / given out and a new ip address needs to be released.

Then after setting up the virtual Server.  I tested joining a windows 7 virtual client to the server 's domain.  The client gets an ip address of 192.168.1.50 with gateway 192.168.0.200 and DNS server 192.168.0.20.
Now I can join the domain, but cannot ping the gateway 192.168.0.200.  So if I get a laptop or smartphone that gets an ip address from the server they will not be able to browse internet.

I think I am just missing something with my DHCP configuration.  Please assist so that I can sort out the problem on the virtual and the live server
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Expert Comment

by:tankergoblin
ID: 39988726
can you dhcp server ping to gateway .200?
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Author Comment

by:Lourensvd
ID: 39988747
Yes the server can ping 192.168.0.200 and is able to browse internet.  Basically every PC that I configure in the 192.168.0.x range is able to ping the gateway 192.168.0.200, but as soon as it goes to a new range 192.168.1.x then they are not able to ping 192.168.0.200
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Expert Comment

by:tankergoblin
ID: 39988753
In your client pc is your subnet 255.255.0.0 and gateway 192.168.0.200??
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LVL 7

Expert Comment

by:tankergoblin
ID: 39988756
try to ipconfig to see your subnet and gateway are correct
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Author Comment

by:Lourensvd
ID: 39988767
Yes, subnet mask = 255.255.0.0 and gateway = 192.168.0.200
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Expert Comment

by:tankergoblin
ID: 39988778
in your superscoping did your change your subnet to 255.255.0.0
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Author Comment

by:Lourensvd
ID: 39988790
I just configured it as new scope with subnet 255.255.0.0.  I did not add a superscope.
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Assisted Solution

by:Zephyr ICT
Zephyr ICT earned 250 total points
ID: 39988828
Can the gateway reach the new subnet? Does it have the larger subnet in its routing table?
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Author Comment

by:Lourensvd
ID: 39988867
Sorry I see that it was my mistake.  I did not configure the gateway 192.168.0.200 with subnet mask 255.255.0.0.  Changed subnet mask on gateway and it worked after I restarted the client.  I can ping from my gateway to the client and the client can ping back and now the client can also browse internet.  
Is there anything else that I need to configure on the gateway regarding the larger subnet or does the gateway do this automatically.
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LVL 25

Expert Comment

by:Zephyr ICT
ID: 39988902
Gateway should be enough normally, unless you have static routes configured, should double-check that to be sure.
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Author Closing Comment

by:Lourensvd
ID: 40003853
Best Solution when your Class C network runs out of IP addresses:  Use Class B subnet 255.255.0.0
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