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Two DHCP servers in same subnet

Posted on 2004-08-14
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What will happen if I put 2 DHCP server in the same subnet? If I connect new computer to the network witch DHCP will reply first ?
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Question by:AlexC77
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by:pseudocyber
ID: 11803142
Which one will reply first is a tossup.

If you want to have 2 dhcp servers, make sure their scopes don't overlap.
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kpmas earned 250 total points
ID: 11803939
You are playing with fire... :)

When a client fires up their computer with a dhcp client, they will send a dhcp request to 255.255.255.255 (broadcast) asking for any available DHCP servers to reply.  The client will get two answers (one from each dhcp server because it can see both) and take the *last* answer it receives.  Causes mass confusion inside a network as you'll have *no* way to control who gets which IP address.

The only method around this is to use MAC authentication where your client mac addresses are filtering against each DHCP server.  If a MAC address exists in the local database, then it will answer and provide an IP.  If it doesn't exist it'll ignore the request.

Why do you want two different dhcp servers in the same subnet anyways if you don't mind me asking?

Thanks,

Paul
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Expert Comment

by:Robert_Ozone
ID: 11804014
Define scopes that don't overlap
Example:
DHCP #1
10.0.0.1-101

DHCP #2
10.0.0.102-254
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by:kpmas
ID: 11804170
That will not work... those scopes are not valid subnet ranges anyways....

The clients will still make a request and besides some of my comments above, you have no way to control which scope each client will go into...

Presuming the question was on how to control who goes into each scope?  Perhaps I misunderstood the actual question..... but then why wouldn't you just make a scope larger than adding a second DHCP server??

Paul
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Expert Comment

by:jmitkin
ID: 11804237
It depends. First of all try that their scope to not overlap. If you have a Win 2000 or 2003 domain you simply can't have 2 DHCP servers because only one have the authority to lease address.
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by:AlexC77
ID: 11804584
2 kpmas: The reason why I want 2 DHCP servers is because I have office computers and production computers. I want one DHCP server assign IP addresses to office computers, another DHCP server to production computers. Two different subnets doesnt work in my case, I need all the of computers to be in one subnet. And you are right, mu actual question was how to contol who will take IP from first server and who from second.

2 Robert_Ozone: Of course I will have 2 different scopes, each DHCP server will assign IP different IP's. But from the same subnet. For example 1 DHCP: 192.168.0.1-192.168.0.127, 2 DHCP: 192.168.0.128-192.168.0.254. Subnet mask will be 255.255.255.0 on both of them, so all computers will belong to the same subnet.
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by:LimeSMJ
ID: 11804666
Your only option is to VLAN the two DHCP servers from one another on your switch (if your switch is capable of that)...  but it would be much easier to just setup two subnets since if you were to VLAN, you need to setup rules as to what packets are allowed from one VLAN to the other VLAN.
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by:kpmas
ID: 11804684
Hi AlexC77...

Is there a particular reason you want to have your office and production computers on the same subnet?  Sorry if I'm harping on this, but if it's a traditional Windows network why not subnet two different sections off which will solve your DHCP requirements?  As long as you have a WINS server somewhere in the network, you could run netbios functions over IP if needed.... a lot of different ways to accomplish this *if* it's possible to split them..

Failing that, the only other solution I can think of is to use Mac authentication on the DHCP servers.  I haven't done this on a Windows DHCP server in quite some time but in Linux DHCP you can permit only certain Mac's to obtain an IP address.  So, add a list of each one and then only those servers can service those clients etc...

Hope this helps..:)

Paul
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by:AlexC77
ID: 11804705
Thanks for all your time.
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by:Ghousullah
ID: 12056022
Pl let me know whether You have what domain you have?....Because in Windows 2000 DHCP servermust be authorised by the AD.
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