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DHCP Scope Conflict

Posted on 2007-04-03
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Last Modified: 2012-05-05
Hello All- I am attempting to set up a new DHCP scope and encountering the
error message "The address range and mask conflict with an existing scope."
The scope I am attempting is 192.168.150.210-234. The existing scopes are on
two different servers: Server1 Scope 192.168.150.20-74, and Server2 Scope
192.168.150.75-124. All scopes are /24. I was able to set up the two existing
scopes with the same mask, but do not understand why an additional scope on
Server2 (Windows 2k3) is producing this error. Any ideas why I am encountering this error?

Thanx

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Question by:ahrimann
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6 Comments
 
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Expert Comment

by:LimeSMJ
ID: 18847563
By design, Windows DHCP can only serve one scope per subnet.  The reason why the Server1 and Server2 live happily on the LAN with the same subnet is because there is no overlap... whereas trying to add additional IPs to Server2 on the same subnet violates the "one scope per subnet rule".  What you are trying to do must be done using IP exclusions within the scope on Server2.  

On Server2, you need to change the original scope to encompass the start and end point for the DHCP scope space you desire:  Start would be 192.168.150.75 and end would be 192.168.150.234.  You would then add a rule to exclude 192.168.150.125 to 192.168.150.209.  Note: It is best practice to deactivate, delete and create a new scope instead of just increasing the address space on an existing scope, otherwise your clients may experience NACKs on DHCP requests.
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Author Comment

by:ahrimann
ID: 18851828
Thanx for the heads up. If I wanted to distribute these excluded addresses (.210-.234) to a specific group of hosts (IP phones actually), how would I go about doing that?
Thanx
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LVL 7

Accepted Solution

by:
LimeSMJ earned 250 total points
ID: 18854388
Real easy... Setup DHCP Reservations.

All you would need to do is find out the MAC address of each phone and reserve them positions in your scope.  Just do a Create Reservation and everything is self-explanatory within the window that pops up.

Just another tip... if the phone doesn't have the MAC address listed on a label somewhere, just plug it into your network and in your Address Lease list, you can find the MAC address that way.  Just remember that once the Reservation is set, you will have to power cycle the phone (or just disconnect and reconnect it from the network) to get the new Reserved IP.
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LVL 7

Expert Comment

by:LimeSMJ
ID: 18854411
Oh I forgot to mention, if you have more IPs than phones and want to restrict that IP space exclusively for phone usage, a simple way to do it is by assigning a local MAC address (just a random hexadecimal with length 12 BUT the last digit is 1... ie: ab-04-00-c3-00-01).  Chances of you typing in a actual working MAC address on your network is 1:281,474,976,710,656.  :)
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Author Comment

by:ahrimann
ID: 18854467
Thanx LimeSMJ- I did figure this out, but I was hoping there was a way to do it dynamically. Ah well, more job security I suppose...
;)~
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Expert Comment

by:LimeSMJ
ID: 18854490
I wish there was too.  Someone needs to make a utility so that you can just right click on an existing lease and convert it to a reservation.

Thanks for the points.
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