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Ian Boyd
Asked: 2018-10-13 07:42:25 +0800 CST

How to add another address range to DHCP server

  • 772

Short version

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Long Version

Window Server 2012 DHCP server is currently configured as:

  • Subnet: 10.0.x.x (e.g. 10.0.0.0/16)
  • Dynamic IP Range: 10.0.0.12 - 10.0.0.120

enter image description here

This is done to limit dynamically assigned addresses to a small range on the subnet (and not overlap into other ranges).

I would like to add another address range to pool of available ranges that DHCP can pull from on our 10.0.x.x subnet:

DHCP Address Pool

  • 10.0.0.12 - 10.0.0.200
  • 10.0.27.12 - 10.0.27.150

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Of course you can't do that because:

enter image description here

Of course it doesn't conflict; it's just being stubbon.

I want the DHCP server to offer addresses from a pool of two ranges on the 10.0.x.x subnet:

DHCP Address Pool

  • 10.0.0.x
  • 10.0.27.x

How do i do that?

Workaround

i suppose i could do it with exclusions:

  • Range: 10.0.0.0 - 10.0.255.255
  • Exclude: 10.0.0.0 - 10.0.0.11
  • Exclude: 10.0.0.151 - 10.0.0.255
  • Exclude: 10.0.1.0 - 10.0.26.255
  • Exclude: 10.0.27.0 - 10.0.27.11
  • Exclude: 10.0.27.151 - 10.0.26.255
  • Exclude: 10.0.28.0 - 10.0.255.255

But that idea is so dumb that I won't even bring it up.

Bonus Reading

Bonus Chatter

How does DHCP work?

Dynamic host configuration protocol can be used to automatically configure network devices with information they need:

  • Network subnet mask (e.g. 255.255.0.0)
  • DNS server (e.g. 10.0.42.7, 10.0.13.29)
  • Domain name (e.g. stackoverexchange.com)
  • Default gateway (e.g. 10.0.241.1)

In addition to being able to configure host options, it can automatically give clients an IP address. The DHCP server is given a block of IP addresses it can assign to clients, e.g.:

  • 10.0.0.12 -10.0.0.100
  • 10.0.0.200 - 10.0.0.245
  • 10.0.3.100 - 10.0.3.200

And when a client shows up needed an IP address, it looks in it's available pool of addresses, picks one, and gives it to the machine:

  • 10.0.0.12 - Unassigned
  • 10.0.0.13 - Unassigned
  • 10.0.0.14 - Unassigned
  • ...
  • 10.0.0.98 - Unassigned
  • 10.0.0.99 - Unassigned
  • 10.0.0.100 - Unassigned
  • 10.0.0.200 - Unassigned
  • 10.0.0.201 - Unassigned
  • 10.0.0.202 - Unassigned
  • ...
  • 10.0.0.243 - Unassigned
  • 10.0.0.244 - Unassigned
  • 10.0.0.245 - Unassigned
  • 10.0.3.100 - Unassigned
  • 10.0.3.101 - Assigned to de-ad-be-ef-ba-ad (IANBOYD)
  • 10.0.3.102 - Unassigned
  • ...
  • 10.0.3.198 - Unassigned
  • 10.0.3.199 - Unassigned
  • 10.0.3.200 - Unassigned

I want to add more ranges of IPs to the available pool of IPs to be assigned.

Other DHCP servers do it

Of course other DHCP servers can do this:

enter image description here

But i already know how to do it in other DHCP servers. I'm asking how to do it in the DHCP server that ships with Windows Server 2012.

If Windows Server 2012 DHCP server cannot do it: it's ok to say it:

It cannot be done; Windows Server 2012 does not support this feature that other DHCP servers support.

But i'm hoping it does support it. DHCP has been around a long time; and Microsoft has had a long time to get it right.

windows

2 Answers

  1. Of course it doesn't conflict; it's just being stubborn.

    It does conflict with your existing scope. Your proposed new scopes are encompassed by your existing scope. In order to do what you want you would need to change the subnet mask of your existing scope so that it's range doesn't encompass the proposed new scopes.

    Addendum

    Based on your comment to my answer:

    I don't know anything about any other DHCP implementation, but it's doubtful to me that any other implementation could do what you want. Saying that it's a "Windows" shortcoming is probably erroneous. How would the DHCP server know to assign an address from your second scope, which is encompassed by your first scope, instead of assigning an address from the first scope? How would it make that differentiation? I'm honestly curious to know how that would work, how you think it should work, and if any other DHCP implementation supports such a configuration.

    • 10

  2. So, your approach depends on the answer to one critical question.

    Are you trying to create two different subnets that exist within one VLAN / broadcast domain? Or are you trying to create two ranges within the same subnet? That is, how many default gateways exist on this VLAN / broadcast domain?

    So, if you have two subnets within this VLAN / broadcast domain:

    1. I assume you need the subnets to be 10.0.0.0/24 (mask of 255.255.255.0) and 10.0.27.0/24 (mask of 255.255.255.0), with the appropriate exclusions
    2. Fix the subnet mask of the existing scope to match the router's network settings.
    3. Convert the existing scope to a superscope, and add a scope within the superscope, for 10.0.27.0/24.
    4. Configure any exclusions necessary.

    Alternatively, if you have one large subnet within this VLAN / broadcast domain:

    1. Set the range for the scope to cover the entire possible range (10.0.0.0 - 10.0.255.255)
    2. Create an exclusion for 10.0.0.1 - 10.0.0.11
    3. Create an exclusion for 10.0.0.201 - 10.0.27.11
    4. Create an exclusion for 10.0.27.151 - 10.0.255.255

    If desired, you can set the range for the scope to cover only 10.0.0.12 - 10.0.27.150, which would mean you could create a single exclusion for 10.0.0.201 - 10.0.27.11.

    • 3