Running Windows 2008 DHCP with 2 seperate scopes on the same physical network

resolver1
resolver1 used Ask the Experts™
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Hi ,

I'm in the process of changing our company internal network from a public IP scheme to a private IP scheme.  The reason for the change is that im installing a new windows 2008 sbs server as the domain controller replacing a linux DHCP server.  How do i get windows 2008 DHCP to give the old IP scheme to specified clients and for all the other clients give them the new IP scheme?

Here's what ive tried so far.  I have setup 2 scopes in DHCP below:

Old public scheme on 192.34.98.x
New private scheme on 192.168.50.x

I've noted all the clients MAC addresses which need to be on the old IP scheme and entered reservations under 192.34.98.x scope.  However, the clients with reservations still receive ips from the 192.168.50.x scope, any ideas why? all clients are on the same physical network.  

If the clients dont have a IP reservation under 192.34.98.x i want them to receive a 192.168.50.x scheme.  How can i achieve this?

Any suggestions are welcome.
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JamesSenior Cloud Infrastructure Engineer

Commented:
Configure the lease on the 2 scopes this should work and also the range on the 2 scopes.

Author

Commented:
by lease, do you mean reservation?
JamesSenior Cloud Infrastructure Engineer

Commented:
Yes.
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Author

Commented:
I'm sorry, I'm a little confused, I can't configure the lease on the 2 scopes. I've added the reservation to 192.34.98.222 to scope 192.34.98.x, it works fine.  However, when I add reservation 192.34.98.222 to scope 192.168.98.x i receive the error "the specified DHCP client is not a reserved client".

Why would I want to configure the lease to the 2 scopes anyway? If the reservation is on 192.34.98.x scope i want dhcp to give the client its IP from the reservation.  If there is no reservation on this scope, I want it to give a lease from the 192.168.50.x scope.

Commented:
I'm not sure if you'll be able to get this to work.

Having two DHCP servers advertising different subnets on the same physical network is going to cause the issues you've described.

The only suggestions I can think of, is investigating the use of Network Access Protection to restrict who is able to get an IP from the scope or having two DHCP servers and use MAC filtering to prevent them from obtaining a DHCP lease from the "wrong" DHCP server.

Both of these options (particular the NAP option) require a fair amount of configuration, so therefore may not be a viable solution.

Author

Commented:
We will have only one Windows 2008 DHCP server that will have the 2 scopes.  There will be no other DHCP server on the network.
JamesSenior Cloud Infrastructure Engineer

Commented:
In DHCP the default lease time is 8 days. What you need to do is on the one of the PCs that you created a Reservation on the DHCP Server, is open the command prompt and then type ipconfig /release and then press enter and then type ipconfig /renew. See does the PC get the new IP address that you assigned on the DHCP Server. If this does not work, try clearing ARP cache on the PC. You can do this by right clicking on the Local Area Connection and select Repair, of from the command prompt type arp -d.

Author

Commented:
Here's a table showing an example client showing the networks it was connected too and the reservations entered in the DHCP scopes and the ip received from the DHCP.  As you can see as soon as a reservation is entered in 192.34.98.x scope, it takes presedence over the 192.168.50.x scope.  

        Address  Given   Client connected to LAN 50.x  client connected to LAN  98.x   Reservation on 50.x   Reservation 98.x    192.168.50.110  yes  no  192.168.50.110  no    192.34.98.2  yes  yes  192.168.50.110  no    192.34.98.250  yes  yes  192.168.50.110  192.34.98.250     192.34.98.250  yes  yes  no  192.34.98.250     192.34.98.250  no  yes  no  192.34.98.250                

Author

Commented:
If the clients dont have a IP reservation under 192.34.98.x i want them to receive a 192.168.50.x scheme.  How can i achieve this?

Author

Commented:
heres the spread sheet with the details.
DHCP-lease-details.xlsx
Senior Cloud Infrastructure Engineer
Commented:
What you need to do then is create a range for the PCs that you have allocated Reservations, Let say you have 50 PCs and you want the 50 to have reservations, so create a address pool say 192.168.1.1 - 192.168.1.50 ok? The next scope which is for the other PCs say 192.168.2.1 - 192.168.2.50, then the other group of PCs will start using this range.
JamesSenior Cloud Infrastructure Engineer

Commented:
What I was trying to explain to you in my previous comment is that when a PC is given an IP address is holds on that IP address for 8 days which is the default period. It was not try to renew it's lease until half through it's lease. That's why I suggesting that you release the ip address and then renew the ip address ok?

Author

Commented:
Thanks for your replys, its a apreciated!

OK, i think i understand you.  Let me re-phrase it in my own words:  

Make sure all IPs in 192.168.1.1 - 192.168.1.50 scope have reservations.  Then the DHCP server will relise there are no more ips to give out in that scope so it will start to give out ips in the 192.168.2.1 - 192.168.2.50 scope?

I understand about the IP lease time period.  I will instruct all users to switch off their machines at night and then switch it on again in the morning so they are forced to re-contact the DHCP server.  
JamesSenior Cloud Infrastructure Engineer

Commented:
Correct!
JamesSenior Cloud Infrastructure Engineer

Commented:
You can also set exclusions on the addresses that you are using for Reservations so DHCP will not give these addresses as oppose to creating 2 scopes. This is the other option you have.

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