help me explain

Cuttler, Brian R (HEALTH) brian.cuttler at
Mon Oct 24 16:05:48 UTC 2016


I have performed testing and I am in fact running 4.2.5 which is anomalous regarding requirement for zone config statements.

Thanks for your help, you are confirmed write, except for my site which is always an oddity.


> -----Original Message-----
> From: dhcp-users [mailto:dhcp-users-bounces at] On Behalf Of
> Simon Hobson
> Sent: Monday, October 17, 2016 2:32 PM
> To: Users of ISC DHCP <dhcp-users at>
> Subject: Re: help me explain
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> On 17 Oct 2016, at 16:54, "Cuttler, Brian R (HEALTH)"
> <brian.cuttler at> wrote:
> > We are in process of restructuring our network in one of our buildings.
> There are good aspects of this, better redundancy, dual paths from each
> switch to the primary router on site, etc, and there are parts of this
> that are not strictly necessary and will in some ways make more work.
> It's always "work", but if you do it right it's usually worth it
> eventually.
> > We are dividing the 6 floor building from a /21 network and creating a
> > new /24 on each floor, we are for the first time in this building
> > enabling DDNS. (yes, we have a net of two /24 networks free when we
> > are done)
> >
> > The positives are that the printers will now provide an identifier
> (string matching their inventory tag) to DHCP then to DNS, and we will be
> able to create DNS short names pointing to their FQDN, so we don't need to
> remap anything from either the print servers or directly mapped printers -
> well, for printers mapped by ID rather than IP.
> >
> > I know from when we did something similar at the first building which is
> using a /22 network for the entire building (regardless of floor) that I
> can use a single subnet name, and can have one named Forward table but
> needed 4 Reverse tables. No problem there. (Is there a better/easier way)?
> Yes, that's correct.
> All the DHCP subnets can share a single forward DNS zone. If they were
> smaller than /24 subnets then you'd also find that some of them would be
> sharing reverse DNS zone as well. The only reason you need separate DNS
> reverse zones is that you can only split the URL at a "dot" - so for
> example if you were using, you'd have to use
>,, and so on.
> You don't need to specify the reverse zone within the subnet declarations
> in DHCP - the server will figure that out automgically, as in a client
> with IP address would automatically trigger a reverse pointer
> update for and if your DNS is correctly setup
> then the DHCP server will automagically figure out the zone to update.
> The main reason for specifying DDNS zones in DHCP is to assign an update
> key for secured updates.
> > The issue in question is that while it is only a /24 on each floor and I
> can use one Forward and one Reverse table FQDN (I believe) needs to be
> unique by floor. IE if a printer moves I don't need to lock it down, never
> need to enter it in DHCP, but do need to change its CNAME to point to the
> new FQDN since each floor requires a different subnet name.
> You don't have to use different forward zones per floor - they can all use
> (say) "" and moving a device around will not
> change it's DNS name. Or, you can choose to use different names, eg
>, and so on. Some people go further
> and have different DNS names for different departments (even if they are
> on the same subnet (eg,
>, ...) That's really a management
> decision, though the technical issues may be part of the input to that
> choice.
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