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Extending my DHCP Scope

Posted on 2013-01-17
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Last Modified: 2013-01-17
Greetings,
 I was hoping someone could help be extend my scope of Ip Addresses. Would This work:

Start IP : 192.168.19.1
End IP : 192.168.20.253
Subnet: 255.255.255.0

Thanks for your knowledge.
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Question by:GeneralDigitalITS
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by:rharland2009
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The mask should be 255.255.254.0 for a network starting at 192.168.19.0 and ending at 192.168.20.255 (I'm including the network and broadcast addresses for the subnet).
The .19 is also a strange place to start a /23 network. Are you contiguous to other /24 blocks?
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by:GeneralDigitalITS
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How could I extend but maintain my 255.255.255.0 subnet. I'm trying to do this without interrupting any network connection. Thanks.
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by:rharland2009
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Yeah, just checked - the .19 won't work for a /23. It crosses the bit boundary
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by:rharland2009
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Can you extend in either direction?
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by:GeneralDigitalITS
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In all honesty I'm not sure, out last IT guy left without any documentaion on this. I guess I know more than others in the building so I'm trying to deal with this, def not an expert though, srry.
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by:GeneralDigitalITS
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The only scope i have right now in 192.168.20.1 - 192.168.20.253 on the 255.255.255.0 subnet
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by:rharland2009
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That's good then....instead, extend upward!
Like this:
192.168.20.0/23

Usable host range: 192.168.20.1-192.168.21.254
Should be noninvasive.
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by:GeneralDigitalITS
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I get "The specified range either overlaps an existing range or is not valid"
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rharland2009 earned 500 total points
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You'll have to nuke and recreate it. It's quick but necessary.
Taken from MS:

Resubnetting
 Resubnetting is the recommended procedure for increasing a DHCP scope when the current scope has entirely consumed the current subnet mask. However, this method requires you to change all subnet hosts and gateways. If you have an address range that has run out of available host addresses, you may be able to change the subnet mask to include a larger share of host addresses. However, simply changing the subnet mask requires that all routers and other statically assigned computers be reconfigured and all DHCP clients have renewed their lease obtaining the new parameters.

Additionally, the entire DHCP scope or scopes must first be deleted and then re-created using the new subnet mask. The potential for duplicate addresses exists during this period if you do not take steps to prevent leasing addresses that other clients may use. Despite all of the aforementioned caveats, resubnetting is still the recommended procedure. The resubnetting configuration creates no additional overhead on the subnet routers or gateways, and keeps all hosts on the same broadcast address.

The following example shows a depleted subnet with the following settings:
Subnet Address: 192.168.1.0
Subnet Mask: 255.255.255.0
 This yields a network of 254 hosts with addresses from 192.168.1.1 to 1921.68.1.254.

The following example shows the result if you use the resubnetting option:
Subnet Address: 192.168.1.0
Subnet Mask: 255.255.254.0
 You now have a network of 510 hosts with addresses from 192.168.0.1 to 192.168.1.254 (for scope 192.168.0.0), or 256 newly available DHCP addresses.
Before:
---------192.168.1.0/24-------R-------192.168.5.0/24---------

After:
---------192.168.0.0/23-------R-------192.168.5.0/24---------
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by:rharland2009
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Just do it after hours. It only takes a minute.
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by:GeneralDigitalITS
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but what about all the printers and and other static devices? I must have atleast 20, they will need to be manually reconfigured, right?
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by:rharland2009
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Any static device will need to have the network mask adjusted, yes.
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by:GeneralDigitalITS
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thank for you help
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by:rharland2009
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No problem. I know it sounds like a pain, but I've done it dozens of times. It's really no big deal.
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by:GeneralDigitalITS
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Will printers will reserved Ip address need to be reconfigured?
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by:rharland2009
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If you mean reservations inside the DHCP scope, then officially yes. However, you can export your DHCP config and re-import to save reservations.

Here's one guide to exporting reservations:

http://andrewmorgan.ie/2008/05/26/export-dhcp-reservations-with-mac-addresses/
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