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Increase size of DHCP scope?

Posted on 2016-09-20
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Small network with 1 Server 2008 Enterprise R2 box ... has been increasing slightly throughout years ... adding a few ip phones and ip cameras and need a little more than the standard /24. I will consider vlans later. Right now, what is the easiest way without disruption to my network to increase DHCP scope from 192.168.1.0/24 to 192.168.1.0/22? Of course, still keeping my current reservations and possibly current leases? Thx for help..
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Question by:xav1963
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by:Schnell Solutions
Schnell Solutions earned 250 total points
ID: 41807350
The less impact thing that you can do is to combine within the same physical network more than one VLan. Right now you have 192.168.1.0/24. You can for example add 192.168.2.0/24, 192.168.3.0/24, ... and then you create a super scope in the same physical network, so it will combine you all of these networks. In this way you do not need to affect your current users, leases, devices configuration, etc and just work normally. (It will require to add that configuration in your router or l3 switch. Alternatively, in the same way you can mix that network with a super scope using a different bigger network, as far as it do not collide with your existing network. (not necessarily a sequence).

A different option could be to change the scope completely like you propose, but it will imply changes in devices or servers (if any) with manual IPs, and a renewal of your client's current IPs. Nonetheless, you will not retain your current leases, if you want to avoid conflicts after such a change you can exclude the ip ranges from 192.168.1.0-192.168.1.255, them from the new pool you will have more addresses.
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by:xav1963
ID: 41807419
I will tackle vlans later ... client needs to upgrade some switches first. If I understand correctly - will this work?
1. Delete current 192.168.1.0/24 scope.
2. Create new 192.168.1.0/22 scope.
3. Exclude 192.168.1.1 to .254 range.

This way I have time to update manually the static clients, right? Will everything still run the same - dhcp, reserved and static? Will the reserved automatically be erased when I delete the original scope? Is there a way to export/import the reserved, even though they have a different subnet mask?
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by:Schnell Solutions
ID: 41807696
Yes, It will work from dhcp clients perspective. Be aware that if If other devices are set with manual ips you need to reconfigure them as well.
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Craig Beck earned 250 total points
ID: 41807899
You don't need to create a superscope.  You only want to do that if you have multiple IP addresses on the server's NIC.

Increase the DHCP scope to a /22 and change the server's subnet mask to 255.255.252.0.

This gives you addresses from 192.168.0.1 to 192.168.3.254.
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by:Schnell Solutions
ID: 41808898
Hello Craig,

Just for not confusing other potential people reading this blog. Superscopes can be used with multiple IP addresses on the same server's NIC, but it is not than that is the 'only' way to use it, it is even not the common way to use it, as far as this configuration is usually set on routers and not on the server.

There are multiples ways and purposes of configuring superscopes, and it is not mandatory to put multiple IP addresses on the same server's NIC. In the practice, when superscopes are used the additional addresses are usually handled by the Router within the same physical network, rather than the server, some examples, scenarios and concepts of superscopes are listed here: https://technet.microsoft.com/en-us/library/dd759168(v=ws.11).aspx.

Another detail, just for clarifying, it is not just to increase the DHCP scope to /22. The scope does not increment as far as an expansion that involve a subnet mask change is not possible. It requires deleting it and re-creating it. And as specified before, it requires reconfiguring the server's ip addresses and probably the one of communication devices involved in the network as well. (Process specified by Xav1963).
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by:Craig Beck
ID: 41809058
@schnellsolutions - Thanks for the pep-talk.

Unfortunately your statement isn't quite right.  Superscopes are specifically for where you use multiple IP addresses on the DHCP server's NIC, or where you're using multinets.  It is EXACTLY where you use it and is extremely common.

See Microsoft's explanation, here...
https://technet.microsoft.com/en-us/library/dd759168(v=ws.11).aspx

I never once said it's mandatory to put multiple IP addresses on a server's NIC in order to use superscopes.  What I said is that you'd use a superscope where multiple IPs were on a server's NIC.

Here, you wouldn't want to use superscopes.  You'd simply create one scope for the subnet you're going to implement.  If you were to add multiple IPs to the server's NIC you would use a superscope.

Also, I really don't need teaching how to subnet.  I do it day-in, day-out.  I can't really understand your last paragraph properly, but it is entirely possible to simply change the subnet mask on the server and the router, and create a scope with the correct subnet size.  You absolutely and categorically DO NOT have to change the IP address on either the server or the router in order to use a /22 or even a /23 subnet size.
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by:Schnell Solutions
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Thanks Craig, it looks that you did not get my points, indeed. Like I said, it is for avoiding confusion to possible readers of this forum...

Because you wrote on your first comment: You only want to do that if you have multiple IP addresses on the server's NIC. At least you corrected it for the second comment, because now you added: or where you're using multinets. That 'OR' is important in the comment :).

This is because the point is not 'server's NIC, the point is multiple IPs on the same physical network (They can be configured at the router interface for example). It is not something necessarily 'only' on the server. You can review it on the same link that you sent as well.

I have administered implementations with the use of superscopes with Windows NT, 2003, 2008 and 2012. And none of them required multiple IP addresses on the server's NIC. That is the point that I wanted to clarify.

My last paragraph was because you wrote Increase the DHCP scope to a /22. It does not sound like changing the subnet mask of a server or a router, as far as it says DHCP scope it means that it is the DHCP configuration. And to make it clear... One scope cannot simply be increased, it needs to be deleted and re-created again. But again... you made the correction on your last comment specifying that they are the changes on the server and router subnet mask. You last comment is fine. But please, check what you wrote on your first comment. That is what I was talking about, but now you made it clear  :).

By the way... the link that yo sent also specifies that one scenario for the use of superscopes is when:

The available address pool for a currently active scope is nearly depleted, and more computers need to be added to the network. The original scope includes the full addressable range for a single IP network of a specified address class. You need to use another range of IP addresses to extend the address space for the same physical network segment.

But anyway, like we are stating, the other option can be carried out as well.
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by:Craig Beck
ID: 41809334
Hmmm, no... I got your points.  I merely added further clarity in my second post.

It appears that it was actually you who didn't understand my comments.  You're implying that I think if we change a subnet mask we change the scope size.  That is simply incorrect.  Also, I'm being specific to this scenario.  We obviously have a single VLAN so the DHCP server is on the same segment as the clients.  That is why, in my first post, I said...

You don't need to create a superscope.  You only want to do that if you have multiple IP addresses on the server's NIC.


Anyhow, you wrote...

My last paragraph was because you wrote Increase the DHCP scope to a /22. It does not sound like changing the subnet mask of a server or a router, as far as it says DHCP scope it means that it is the DHCP configuration.

However, I clearly said...

Increase the DHCP scope to a /22 and change the server's subnet mask to 255.255.252.0.

You see, I said we need to increase the DHCP scope size AND change the subnet mask.  Perhaps you missed that?  I therefore didn't need to correct anything in my first or second post as I was correct the first time in both cases.  The only adjustment I may have made is to substitute the word SPECIFICALLY for ESPECIALLY in my second post.

Well done for picking out the section that states you can superscope when address pools are depleted.  Unfortunately though, you failed to understand the assertion that here we also need to add multiple IP addresses to the server's NIC in order to allow the server to be able to talk to clients on those subnets.  I'll let you work out why, but I'll give you a clue.... we're on the same layer-2 segment.  

To be more specific... you are right that it isn't mandatory to add IP addresses from each subnet on the DHCP server's NIC in order to give IP addresses, but I also clarified that earlier in response to your incorrect statement.

The point is that if your server has 192.168.1.1/24 and you give a client 192.168.2.76/24 it won't be able to see the server without using a router.  That's basic IP because of the mask.  Therefore you have to give the server an IP address in each of the subnets that you're assigning when you're on the same layer-2 segment, or clients on the new subnets won't talk to the server.
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by:Schnell Solutions
ID: 41809513
It is so simple... You used two sentences that are confusing (for not telling than incorrect).

Sentence 1: (You are stating on the first sentence...)
You only want to do that if you have multiple IP addresses on the server's NIC.

There you were missing than that is not the only case like that sentence says. it says 'only' according to what you wrote there, and it is NOT 'only' on the server NIC, it is even more commonly configured in a router. Is it to complicated to get the point?

Sentence 2: (You are stating on the second sentence...)
Increase the DHCP scope to a /22 and change the server's subnet mask to 255.255.252.0.

It is more clear to specify that this increase consist on deleting the subnet and re-creating it again. Tomorrow someone can read this and might just try to modify the existing scope with the subnet that it uses, when that is not possible. Of course, it looks like that you mean deleting, recreating it. But I just mean to clarify it.

If you do not want to make an effort to understand what is confusing with these two simple sentences that you wrote and understand that it can confuse others; or if it is the case that you just want to justify somehow than it is clear and correct making circles over the same thing... that's another point.
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by:Craig Beck
ID: 41810841
Please don't talk to me about simple sentences and confusing others...

There you were missing than that is not the only case like that sentence says.

What it that nonsense???!?!??!!

If YOU can't understand what I said, that's your issue!

Sentence 1: (You are stating on the first sentence...)
You only want to do that if you have multiple IP addresses on the server's NIC.

Yes, that's absolutely correct in THIS scenario.  Why are you apparently only generalizing when it suits your argument?

it says 'only' according to what you wrote there, and it is NOT 'only' on the server NIC, it is even more commonly configured in a router. Is it to complicated to get the point?

We're not talking about a routed network here though, so what IS your point?  We're talking about THIS question.

Really buddy, there's nothing you could teach me about IP networking and networking technologies on a server or a router.  I've probably forgotten more than you'll ever learn.
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by:Schnell Solutions
ID: 41810901
God bless you my dear.   :)
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by:xav1963
ID: 41811268
well... ok .... it seems that I have a choice here, depending what I consider easier.
Choice 1 is add additional subnets, leaving original in tact, add superscope to encompass all subnets, add ips to server, configure router. No need to change current clients.
Choice 2 is delete existing scope, redo with /22, as time permits -change subnet mask on server and static clients. No need to reconfigure switches or router.

Does that sound about right? I think  I am going with Choice 2.
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by:Schnell Solutions
ID: 41811300
Yeap, that is right.

But it would be nicer if you additionally set the router interface as well to match /22. Because even if it can work (depending on your router configuration) that will corresponds exactly to the same network ID that you are using.
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by:Craig Beck
ID: 41811653
No that's not right. Option 2 requires you to change the subnet mask on the router. You MUST do that or clients using the new IPs won't be able to get off the network. You need to change the masks from the start, not as time permits.
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by:xav1963
ID: 41868507
Thanks for your input.... appreciate it greatly...
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