Multiple subnets on one Interface

Glenn Satchell glenn.satchell at
Tue Jan 28 06:19:42 UTC 2014

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.


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