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IP addressses limit on DHCP

Posted on 2012-03-15
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Last Modified: 2012-06-27
I have 3 DC's (2 of which are DHCP) Servers on my LAN as follows:

DHCP Server 1 has an address pool of 10.1.20.128 to 10.1.20.254
The DHCP icon has a yellow excclamation point on it and in the app log I have errors that it is 91% in use and only 12 addresses left.

DHCP Server 2 has an address pool of 10.1.20.16 to 10.1.20.127
No errors on this one.

Why does the 2nd dhcp server have much less leases that the first one? Do I need to create a new vlan and DHCP server for more IP's for the users?
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Question by:tolinrome
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8 Comments
 
LVL 43

Expert Comment

by:Adam Brown
ID: 37726746
If both scopes are on the same network, you need to bind them together with a Superscope in order for the DHCP server to hand out leases from both.
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LVL 43

Expert Comment

by:Adam Brown
ID: 37726751
Actually, wait a minute. I misread that. Thought both scopes were on the same server.
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LVL 41

Expert Comment

by:Kyle Abrahams
ID: 37726794
What is the number of total users?  

Example of a setup using split scope:  
http://technet.microsoft.com/en-us/library/ee405264(v=ws.10).aspx

Mostly everything I've read says use the 80:20 rule, but 50/50 makes more sense to me for failover scenarios.

Can you create a supernet?


EG:
10.1.20.16 to 10.1.20.254 on server 1 and
10.1.21.16 to 10.1.21.254 on server 2
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LVL 7

Author Comment

by:tolinrome
ID: 37726878
it's the users (150 +/-) combined with iphones and ipads etc. I'd have to read up on a split scope and supernet to see what that means. I'll have to get back to you.
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Expert Comment

by:Martin81
ID: 37726978
What's the subnet mask of your network/networks?
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LVL 96

Accepted Solution

by:
Lee W, MVP earned 1000 total points
ID: 37727345
DHCP answers as soon as it can... if one server is "faster" than the other, then it answers more often.  You'd either have to subnet, or just live with it.  If a server failed, the other would answer anyway and it wouldn't matter if one originally gave out 90% of the addresses and the other only 10%.  So long as the addresses don't overlap in the scopes.
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LVL 44

Assisted Solution

by:Darr247
Darr247 earned 1000 total points
ID: 37727452
> 91% in use and only 12 addresses left.

When that one runs out, the other one will be the only DHCP server offering new requests an IP, whether it's usually the fastest to respond or not.
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LVL 3

Expert Comment

by:rafter81
ID: 37728309
Are you using UDP helper for these to know what DHCP servers to go to?  Some switches allow priority servers, so check if that is set up.  Its correct that the 1st server to respond is usually the more resourced server.  A superscope would be the best way to go, but you have to get your subnet masks right, so you always have enough addresses to allocate.  The 80/20 rule makes more sense if your looking to just have a short term backup.  But then again 20% of a large subnet could still be enough for your needs...
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