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DHCP and Subnetting

Posted on 2002-06-05
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Last Modified: 2010-04-13
Situation: Setup DHCP and Subnet current network

How do i set up DHCP so that i can have 2 subnets ?

this is what we have now.

10.1.1.x
255.255.255.0

we are growing past the use of this as it only allows for one network

I would like to go to this
10.1.1.x 10.1.x.x
255.255.0.0

I want all of our servers to have static IP Addresses in the 10.1.1.x range

and all the users to use 10.1.2.x as the subnet.
all users should use dhcp in this range

is this the best way to do this ? if so what is the best way to set it up. do i have to go to all the users and change the subnet mask ? how would i set up DHCP to keep all the traffic seperated ?
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Question by:Auerelio Vasquez
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12 Comments
 
LVL 63

Expert Comment

by:SysExpert
ID: 7056837
A couple of options.

1) You should be able to change the DHCP settings to include your Class B rather than A network.

It may require starting from scratch and defining your new DHCP structure.
You can then delete the old DHCP database and create a new one.

I would save the old one just in case and I would use the dhcp tools in the NT resource kit to export all the information in case you need it.

Since the subnet mask is NOT set except for static IP addresses, all the users should get new addresses when they reboot.

All the servers and static IP machines will reqire a change in the subnet masks manually.

Check the MS site for more info on DHCP and perhaps SUPERScope options if needed.

I hope this helps !
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Expert Comment

by:slang9
ID: 7056848
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Expert Comment

by:jmiller47
ID: 7056955
Make sure that your DHCP server is giving out VERY short DHCP leases before changing over to a new scheme. Otherwise, when you will change over to the new scheme,  the clients might still have a lease for 1 or two weeks and will only switch after their lease is up or you manually visit every client to renew.
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LVL 4

Expert Comment

by:Nevaar
ID: 7059186
Question: Do you now have two LAN subnets (two different phyiscal LANs connected by a router)?  Or are you looking to simply re-layout your IP addressing?
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Author Comment

by:Auerelio Vasquez
ID: 7059517
Nevaar,

i only have one LAN that i want to re-layout.
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Expert Comment

by:rmorphis
ID: 7059766
Kind of overkill for two subnets, could bring mask down to 255.255.254.0. How many devices? not sure if your trying to add devices or segment network? If your going to route between two networks, keep your current subnet & mask, and add a 10.1.2.0/24 network. If you just want more devices and no routing, back off on the mask of your current DHCP will work the same, users will get the new config on boot up, if your routing between networks, you need a box to relay dhcp requests, and create scope for each network. Segmenting by clients and servers may not be the way you want to go, usually determined btphsyical or security factors. Should work fine though....
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Author Comment

by:Auerelio Vasquez
ID: 7063846
so on the DHCP i would setup 10.1.1.x - 10.1.2.x and have the mask be 255.255.254.0 correct ? what about the ones i want to keep static  ? the are all over the place. for example i have one server at 10.1.1.84 and one at 10.1.1.111 could i just have the 10.1.1.x be static for the servers, and have the 10.1.2.x be used for DHCP ? also, what problems might this cause because of the firewall ? do i have to define the range on the firewall object ?
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Accepted Solution

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rmorphis earned 400 total points
ID: 7064397
255.255.254.0 would be for two networks of 10.1.0 and 10.1.0, lets go to 252.255.252 if you want .1 & .2
You can do what either if I'm understaning you right. You can define a DHCP scope and enter exceptions for your existing servers if you don't want to readdress them. If you do want to have a "server" subnet and a "client" subnet. You just need to readdress the nodes. You can define the scope however you wish (how many addresses to lease), and keep the leases short if you have a lot of laptops and such. Let me clarify what we're doing, by changing that mask, we're extending your "local" network. That means all clients are considereing .1& .2 a network that will communicate through broadcasts. This really does nothing to manage traffic or divide you network in itself. IF there is router or multihomed server acting as one that will be dividing these two networks, then you want to keep the 255.255.255.0 mask because the networks will need a gateway to talk to each other. You would just need to makke sure your static addresses are on the right network, if there's no router, you would just have to change the mask on your static boxes. Not sure what your firewall is doing, since these are bot internal networks, you'd probably have to change the mask on some firewall rule to identify each network as such. Hope I'm not confusing you more....
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Expert Comment

by:rmorphis
ID: 7064399
whoops...
255.255.254.0 would be for two networks of 10.1.0 and 10.1.1, lets go to 252.255.252 if you want .1
& .2......
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Expert Comment

by:rmorphis
ID: 7064400
here's a great link to messs around with subnet calculations...

http://www.telusplanet.net/public/sparkman/netcalc.htm
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Author Comment

by:Auerelio Vasquez
ID: 7070637
Thanks for the explanation. I think that i can probably just use 10.1.0.0 and 10.1.1.0 so that i can extend the number of nodes. There will not be a gateway between the networks, but i have alot of nodes on the network, and i don't think that i can convince the upper mgmt. team to purchase a server, or router to truly "subnet" but by using 255.255.254.0 will give me 510 nodes correct ? then i can just tell the DHCP server the range and input the subnet mask. If i'm wrong here correct me, the only thing i dont' know, and i'm sure that i can figure out, is the static addresses, can i specify 10.1.1.0 as the range for static entries, and use 10.1.0.0 to be truly dynamic ?
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Expert Comment

by:rmorphis
ID: 7070660
Your correct, give anything static the .0 and define the DHCP scope .1 . Probably already have a dhcp server in place, all you need to do is change the mask on your existing scope. Good Luck!
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