Multiple subnets on one Interface

Steve Clark sclark at
Tue Jan 28 12:19:10 UTC 2014

On 01/28/2014 01:19 AM, Glenn Satchell wrote:
> On Tue, January 28, 2014 9:15 am, Simon Hobson wrote:
>> Steve Clark <sclark at> wrote:
>>> How will dhcpd give out ips? Will it alternate? Give all of one subnet
>>> then start the other
>>> subnet?
>> The official answer is that it is not determinate - as in you cannot in
>> any way rely on any specific allocation order.
>> The practical answer is that *currently* the implementation dependent
>> order is that it will allocate unused addresses starting with the
>> numerically highest and working down - I believe this is due to the way
>> the internal hashing tables work. This is implementation dependent, not
>> officially documented, and liable to change at any time without warning -
>> if you want a specific allocation strategy then you will need to
>> explicitly define it in the config.
> Further, it doesn't really matter, as once it gets to steady-state it will
> be issuing IPs on a least recently used basis, so will essentially be
> random. Exactly the same way it is with a single subnet.
> A common desire it to use fixed-address devices in one of the subnets, and
> a dynamic range in the other. Another option is to use a class to select
> some subset of devices and then use a pool in each subnet, one allowing
> the class and the other denying the class.
> regards,
> -glenn
Thanks to all that answered.

I just wanted to make sure that when all the ips from one subnet were handed out
then dhcpd would start handing out ips from the next subnet.


Stephen Clark
Director of Technology
Phone: 813-579-3200
Fax: 813-882-0209
Email: steve.clark at
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