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DHCP server: Server 2008 R2
IP info: 192.168.0.1/22
Subnet: 255.255.252.0
Gateway: 192.168.1.1

According to every calculator out there, 192.168.0.1/22 means I should be able to assign between 192.168.0.1 through 192.168.3.254 as I see fit. 192.168.0.0 being reserved for routing, and 192.168.3.255 for broadcast, leaving 1022 available addresses.

Currently we can only assign IP addresses between 192.168.1.1 and 192.168.2.254. Scope in our DHCP server has this limit, showing a starting address of 192.168.1.1 and an ending address of 192.168.2.254, allowing essentially only half of the addresses to be used.

  • Can I change the scope to allow addressing of the rest of our calculated range?
  • If so, how do I make this change of scope?
  • Will I need to change the gateway address or can that remain at .1.1?

I would prefer to keep a single scope, if at all possible, keeping all the current exclusions and reservations.

UPDATE:

The main scope on the DHCP server already has the correct subnet (255.255.252.0, or /22). This means I'm not actually changing the subnet at all. I'm "simply" expanding the IP start and end addresses to what they ought to have been initially.

Reading this MS article it seems to answer my question: If I were changing the subnet or adding a new one, I'd have to create a new scope, or delete and recreate. However, since I'm only expanding my current start and end IP addresses to fill the existing subnets usable address space, the correct method should simply be to edit the properties of the scope.

  • Scope properties indicate subnet mask of 255.255.252.0 with a start IP of 192.168.1.1 and an end IP of 192.168.2.254. I believe, despite the limited start and end IPs, this indicates a /22. – music2myear Apr 2 '13 at 15:34
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You can use a single scope for all of those addresses, but according to this discussion you cannot expand an existing scope on MS DHCP, so you need to drop/delete it, and recreate it afresh.

However, if the scope is already that size, you do not need to drop or delete the scope. You can just change the start and end address for the assigned address range, as stated in the article linked by music2myear.

  • Looking at this article, indicates I may be able to though: support.microsoft.com/kb/255999 My existing scope already has the correct subnet (255.255.252.0), it only has the incorrect start and end IPs. Microsoft says I should be able to adjust the start and end IPs without issue. So, professional opinion: will this bollocks up my entire network if I were to simply extend the start and end IPs to their correct numbers given the /22 subnet? – music2myear Apr 2 '13 at 15:47
  • Read the bit about Resubnetting.. – NickW Apr 2 '13 at 15:48
  • I've edited my question above to clarify. In short: I'm not changing my subnet, I'm "simply" changing the start and end IP addresses to fill the usable space of the existing subnet. – music2myear Apr 2 '13 at 16:06
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    Now you've put the question more clearly, I agree with you :) I'll change my answer to include that caveat. – NickW Apr 2 '13 at 16:10
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    Thank you for helping me clarify my own thinking. For a Network Administrator, I consider anything more than the physical aspects of networking to be my biggest weak spot in my IT knowledge. – music2myear Apr 2 '13 at 16:12

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