DHCP leases issue

[Surya Teja]

Hi Simon,
Thanks for reply, Yes I can increase the scope range but i have clients
like 100-120 per floor(per subnet) I don't think it would be good idea to
increase its scope size to some /8 subnet  and over come this issues
Let me explain the scenario. If the client moved from subnet A and subnet B
we may not sure that client returning to previous subnet for that whole
working day or not.
So the IP assigned to that client in subnetA  is of no use for period of
duration. This happens to multiple IP's
I want to set the config in such a way that before DHCP granting the lease
can it check the existing lease file with that mac address and free the
previous ones if it has from other subnets or scope.
As suggested by Thomas I have added the statement *one-lease-per-client
true to config *and till now I didn't see any issues but as it is not sure
I am still observing the cases.

On Sun, Sep 8, 2019 at 4:00 AM Simon Hobson <dhcp1 at thehobsons.co.uk> wrote:

> Surya Teja <suryateja042 at gmail.com> wrote:
>
> > Thanks for reply as suggested i have increased lease time to one hour
> and I observerd one more scenario when the client moves from one subnet to
> another subnet ( lease time say 1hr). The client got IP from the second
> subnet scope but the previous IP in the 1st subnet is still in hold and in
> the lease file. It still recorded an active entry. How can the dhcp server
> reclaims those unused IP's?
>
> You CANNOT do that without violating the DHCP specification. Note that the
> client is within it's rights to store all the leases it has, and on
> returning to the previous subnet, continue using the lease it still has for
> that subnet. So if the server has handed the address out to another client
> in the meantime, you can have an address clash.
> So short version "do NOT do that" !
>
> > The first IP is getting into free state after completing its 1 hour
> lease duration till that time it is active mode only.
>
> That is correct operation.
>
> The correct response to "I don't have enough addresses" is to increase the
> size of the address pool(s). It's a balancing act - on the one hand longer
> leases give you stability and more time to respond to DHCP server issues;
> while on the other hand, shorter leases suit highly mobile users (high
> churn rate). For short leases, even 60 minutes is (IMO) getting rather
> short - you only need one hiccup with your DHCP service and your users have
> between 30 and 60 minutes before they fall off the network and call your
> helpdesk.
>
> If you are finding that you run out of leases then it suggests you have
> your network design wrong. There is LOTS of address space in the RFC1918
> blocks, and you are certainly not constrained to use /24 subnets in the
> 192.168.n.n allocation. Use 10.n.n.n/8 and you have 16 million addresses to
> play with !
>
>
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