3

I am very sorry if this has been answered already. I found this post: DHCP Reservation However I wasn't sure if that was related to Windows or Linux DHCP server and I wanted to clearly state my configuration for an accurate yes/no before I make any changes.

I have a Windows DHCP server with the following config:

  • Scope = 10.10.5.0/24
  • Address pool = 10.10.5.10 - 10.10.5.254

My goal is to limit that address pool to 10.10.5.240 - 10.10.5.254 since non-reserved DHCP clients are very uncommon on this subnet.

However, I have MANY address reservations already configured based on MAC address across the whole /24 subnet and they cannot be affected.

Will adjusting the Address Pool affect my reservations?

3

Well I have pretty much come to a conclusion. In short, yes, it will work (if the reservation already exists). However like so many things from MS there are some caveats since there can be errors if adding a NEW reservation outside of the address pool.

It seems the best method is to utilize exclusion zones on your address pool to limit what range of addresses are automatically assigned via DHCP.

Straight from this Microsoft KB

  • You may create a reservation in any scope range. This can include Excluded ranges.
  • You may create a reservation in a subnet range even if the reservation falls outside the actual scope Start and End addresses for that subnet.
  • You cannot create reservations outside the subnet range of any of your existing scopes.

Using exclusion ranges is preferred because there is a known error that can occur if you attempt to add a reservation that is outside the range:

ERROR:

"The specified DHCP client is not a reserved client"

To avoid this potential issue, use exclusion ranges as defined in this article

If, for some reason, you do not want to/can't set up exclusion ranges, and can't add a reservation outside the address pool, then this hotfix is to solve that bug.

In conclusion / Solution Summary:

Since MS states that reservations CAN be used and assigned WITHIN an exclusion range, what I will do is leave my address pool as-is, and I'll configure the exclusion range 10.10.5.2 - 10.10.5.239

Here is my final config:

Scope = 10.10.5.0/24
Address pool = 10.10.5.10 - 10.10.5.254
Exclusion Range = 10.10.5.10 - 10.10.5.239

This will leave my desired range of 10.10.5.240 - 10.10.5.254 for auto-assigned DHCP addresses and will avoid any potential errors explained above.

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  • 1
    The link you posted doesn't work anymore. – David Klempfner Jul 18 '17 at 22:16

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