DHCP "static" assignments

Gregory Sloop gregs at sloop.net
Wed Aug 7 20:39:40 UTC 2013

SC> On 7 August 2013 20:43, Gregory Sloop <gregs at sloop.net> wrote:
>> [IMO, it would still be a lot easier if you just had to put in the host
>> entry, and then define the pool as 1-20, and it wouldn't assign any
>> IP's for defined hosts to any other host.

SC> So ISC's take is that if it's defined it can lease it, if it's not
SC> defined then it can't be given out, unlike Microsoft's stance of we
SC> can lease it unless you tell me not to.

SC> If you configure a "static" host then that IP will be given to that
SC> host regardless of where it is in the subnet and whether or not it
SC> overlaps an existing pool (if it does overlap the pool then the pool
SC> won't issue that IP address, it ignores it - so in your case above
SC> there is no point in defining a pool when the pool is never going to
SC> get used anyway).

SC> IMHO this is a much simpler model than defining all available space
SC> and then restricting it afterwards, just define what you want to
SC> actually use and ignore the rest.

I see the point, I don't really like how [I think] it works, but it is
what it is. 

But these aren't actually "static" IP assignments - or at least I'm
not sure we're using the same terminology in the same way. [Probably
my fault.]

Here's the effect I want...

I want the DHCP server to hand out specific IP's to specific MAC
addresses, [as defined in the host record.]

They're static DHCP assignments, not static IP clients.
i.e. I want to get assigned *via the DHCP server* to the
host with MAC Address aa:bb:cc:dd:ee:ff

Also, I'm assigning via DHCP addresses from to
anyone who asks via DHCP.

The way I do it now, is assign pools as follows && set "allow unknown-hosts" && set "deny unknown-hosts" && set "allow unknown-hosts"

AND create a host entry for like so:

host somehost {
        hardware ethernet aa:bb:cc:dd:ee:ff;


I *think* I have to include the relevant IP in a pool. (In this case  [If there's no pool with in it, the DHCP server
won't hand out an IP for, no matter what host record there
is, will it?]

Provided I'm right... then...

IF: I include it [the IP Address]  in a pool, and have a proper host
entry, it will still get assigned to ANY host, not just the host
defined in the host entry, *unless* the host defined in the host entry
gets there first.

Thus, to *guarantee* that the host defined in the hosts entry is the
*only* host offered the relevant IP ( in this case) you must
do all three

1) create a pool, with that IP address included, that's set to "deny

2) Define the hosts entry

3) Make sure the "allow unknown-hosts" and the "deny unknown-hosts"
pools don't overlap, etc. [Obvious, but easy to screw up, at least for
me. :) ]

Hope that's clear. Am I doing it right?


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