Question

windows 2003 dhcp over 2 subnets

Asked by: kingcastle

Hi,

I have two different subnets on my lan. I want to be able to service both subnets with one DHCP server. Subnet 1 is x.x.x.x with mask 255.0.0.0 and subnet 2 is x.x.x.x mask 255.255.255.0.

Im using windows 2003 and all my routing is in place and working grand.

Can this be done? and if so how can i set it up so that only pcs on subnet1 get subnet1 ip addresses and scope options and the same for subnet2 pcs

cheers

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Asked On
2007-08-24 at 12:52:27ID22785884
Tags

dhcp

,

2

,

scopes

,

subnet

Topics

Windows 2003 Server

,

Dynamic Host Configuration Protocol (DHCP)

Participating Experts
2
Points
500
Comments
27

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Answers

 

by: vsg375Posted on 2007-08-24 at 13:09:16ID: 19765083

Hi,

This can indeed be achieved, using superscopes. Each scope in the superscope must have the proper DHCP options (DNS, Router, etc), and your router must be RFC1542 compliant (even the most basic routers do that now).

Here's some useful literature about superscopes :

to activate a superscope :

http://technet2.microsoft.com/windowsserver/en/library/eb4e04b4-9c23-4b3e-95c5-abb90c894bae1033.mspx?mfr=true

Superscopes explained :

http://technet2.microsoft.com/windowsserver/en/library/3967ddab-0b28-4959-8b4d-3052c178731b1033.mspx?mfr=true

If you have any prob setting up / activating, please let me know.

HTH
Cheers

 

by: MSE-dwellsPosted on 2007-08-24 at 13:13:37ID: 19765129

Either the router connecting the subnets needs to support RFC 1542 (BootP relay-agent) or you can install a relay agent on one of the Windows servers/clients already on the DHCP server's subnet (I don't like this option much since the second subnet relies heavily on a computer that's more likely to be rebooted according to its own schedule without regard for the impact).  In addition, you could of course connect the DHCP server to both segments by adding a second NIC.

Regardless, the DHCP server will select an address from the correct scope (assuming you've created two scopes suitable for the two subnets) because the router/relay-agent will modify the lease request to allow the DHCP server servicing it to determine which scope to pull the address out of.

 

by: kingcastlePosted on 2007-08-24 at 13:19:51ID: 19765179

thanks for this, so ok i create my main scope x.x.x.x mask 255.0.0.0, i then tried to create my second scope under superscope x.x.x.x mask 255.255.255.0 it stops with an error after i clcik finsih "subnet address and mask is inconsistent with existing one"

cheers

 

by: MSE-dwellsPosted on 2007-08-24 at 13:22:51ID: 19765199

Can you supply the actual scope values, I can't determine the cause without the numbering scheme you're using?  I can, of course, guess but the numbers would help no end.

 

by: kingcastlePosted on 2007-08-24 at 13:25:22ID: 19765220

sorry subnet1 is 10.0.0.0 255.0.0.0 and subnet2 is 10.10.1.0 mask 255.255.255.0

cheers

 

by: MSE-dwellsPosted on 2007-08-24 at 13:26:16ID: 19765228

In addition, and not wishing to argue but you really do not need to use superscopes.  You simply need two scopes suitably configured for the subnets in question.

 

by: kingcastlePosted on 2007-08-24 at 13:28:35ID: 19765246

if i configure two scopes how do i configure it so that only pc's on subnet 1 get subnet 1 scope and subnet 2 pcs only get subnet 2 scope addresses

 

by: MSE-dwellsPosted on 2007-08-24 at 13:29:01ID: 19765251

As you've defined it, subnet1 consumes subnet2 ... thus the error.  Any computer on subnet1 will deem all computers on subnet2 local and refuse to use the router.  This cannot be working as is.  Are these really the subnets in play right now or your interpretation of using superscopes to solve the problem?

 

by: kingcastlePosted on 2007-08-24 at 13:31:38ID: 19765268

these subnet were never physically connected but they are being physically connected now

cheers

 

by: MSE-dwellsPosted on 2007-08-24 at 13:35:13ID: 19765297

The DHCP lease-process employs a number of techniques to solve this problem - first off, the DHCP server interface through which the lease request was received allows it to determine a suitable scope.  This assumes the lease-request has not been modified by a relay-agent (this method doesn't really apply here since I haven't seen mention that the DHCP server is multi-homed).  Second, the relay-agent (which requires a static address) will modify the lease request by adding its own address into the mix thereby allowing the DHCP server to determine the subnet from which the request originated and, subsequently, the scope from which to pull a suitable address for the requesting client.

 

by: MSE-dwellsPosted on 2007-08-24 at 13:36:51ID: 19765307

>> these subnet were never physically connected but they are being physically connected now

Ah, :0(  ... then we seem to have a problem.  Are you in a position to alter the entire subnetting scheme for one or both of these subnets?

 

by: kingcastlePosted on 2007-08-24 at 13:38:33ID: 19765318

yes i dont see this as a problem as long as both subnets can still talk to one another

 

by: MSE-dwellsPosted on 2007-08-24 at 13:43:54ID: 19765346

How many devices (clients, servers, printers, etc.) per subnet?

 

by: kingcastlePosted on 2007-08-24 at 13:46:56ID: 19765368

i can redo subet 2 but not 1 and 2 has about 30 ip addresses in use

 

by: MSE-dwellsPosted on 2007-08-24 at 14:08:02ID: 19765496

That's not a major problem but it does mean we need to alter the entire subnetting scheme since we're compelled to use locally-administered address ranges and subnet1 has consumed the entire 10.0.0.0 range.  Can you not alter the mask of subnet1 such that's 10.0.0.0/16 not /8 (i.e. 255.255.0.0 not 255.0.0.0)?

 

by: kingcastlePosted on 2007-08-24 at 14:09:54ID: 19765502

yip

 

by: kingcastlePosted on 2007-08-24 at 14:10:48ID: 19765510

im signing off now for the night i hope we can pick this up again tomorrow

many thanks so far

 

by: MSE-dwellsPosted on 2007-08-24 at 14:15:32ID: 19765542

If that's the case, then approach it as follows -

subnet1 = 10.0.0.0/16 (or 255.255.0.0)
subnet2 = 10.1.0.0/16 (or 255.255.0.0)

... and create the two scopes accordingly.

Note - you don't have to alter subnet2 in the manner I've recommended, I just prefer to see some kind of order to it and, since you're redesigning, we might as well incorporate some kinf of methodology ... your call of course.

I've assumed you're sufficiently familiar with the IP routing and name resolution requirements and have not included any information related to those aspects of your design.

 

by: kingcastlePosted on 2007-08-25 at 00:49:55ID: 19767119

many thanks for this. just one last thing how do i get the pcs in the relevant subnet to get the correct ip scope

 

by: kingcastlePosted on 2007-08-25 at 01:17:04ID: 19767146

just thinking about this and maybe now is the time to change. if i make my main network 10.1.0.0 255.255.0.0 and my second network 10.2.0.0 mask 255.255.0.0 will pcs on subnet 1 then use a router to access subnet ie subnet 1 will not deem subnet 2 local?

 

by: kingcastlePosted on 2007-08-25 at 02:16:45ID: 19767248

hi sorry about this but its me again, i have had a look and there is no way we can change subnet1 its got to stay at 10.0.0.0 255.0.0.0 but i can change subnet 2 no problem. what can we do is there anyway to get this working?

cheeers

 

by: vsg375Posted on 2007-08-25 at 02:39:02ID: 19767280

Hi,

Europe back online :o)

MSE-dwells is handling the case marvelously, but after a good night of sleep, I feel like adding my 2 cents :o)

the mask of 255.0.0.0 is the widest possible for subnet 1. Therefore, it will eat up any other subnet starting in 10.x.x.x.

If you can't alter subnet 1, the only possibility I see is to use another RFC1918-compliant subnet range, either 172.16.x.x  to 172.31.x.x  or 192.168.x.x. Configure your router accordingly, by adding the proper routes between subnets, define the proper DHCP options in your scopes (especially the default gateway, 003 ROUTER option), and you should be OK.

HTH
Cheers

 

by: kingcastlePosted on 2007-08-25 at 02:49:42ID: 19767307

cheers for this i had thought of using 172.16.1.0 255.255.0.0 for the second network so i am now working on the router to get that side of things going

 

by: MSE-dwellsPosted on 2007-08-25 at 05:41:29ID: 19767664

Per vsg375's comment and my earlier reference, you'll need to use a locally administered address range.  vsg375 outlined your options and your choice appears fine to me too (assuming the router is, of course, configured correctly).

kingcastle: May I ask why you can't change subenet1?  Is it politics, perhaps not yours to change or a technical reason preventing it?

 

by: kingcastlePosted on 2007-08-25 at 06:38:25ID: 19767855

well its really becuase there is lots of static ip addresses on the lan for printers, vpn's etc and our sister company print to our printers over the vpn to ip addresses

 

by: MSE-dwellsPosted on 2007-08-25 at 06:40:47ID: 19767862

Understood.

 

by: kingcastlePosted on 2007-08-25 at 06:43:55ID: 19767871

i actually think that these networks were separate to begin with to ease network load, im not sure if this still will be the case even

cheers

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