Static IP and IP management

[Chuck Anderson]

On Wed, Feb 24, 2016 at 05:04:10PM +0000, Simon Hobson wrote:
> Patrick Trapp <ptrapp at nex-tech.com> wrote:
> 
> > If you are using host entries to dictate what address a device gets (and not allowing devices to grab random addresses - effectively making them static without having to configure it on the device), then when you delete that host entry from the dhcpd.conf, you would know that address is free.
> 
> Yes, but I think the primary issue is knowing that the assignment is no longer needed - as in, that device hasn't been here for a while. Jim has given an example of how I suspect most systems manage it - literally keep track of what IPs and MACs are in use, and see if any of them go stale.
> 
> An alternative approach could be to use reserved leases. That way, each usage of the assignment goes through the normal DHCP lifecycle - including DNS updates. By tracking lease usage etc you can then see if a lease is no longer being used.
> 
> Basically it's the old problem - when something is needed for something else to work then it gets noticed, when that something is no longer needed then it just gets forgotten about.

One other possibility if you can force everyone to use DHCP is just
keep the DHCP logs and look at them from the last time a device
DHCP'd.  That way you can keep using fixed-address assignments, but
managed via DHCP.  It helps if you have switches that support DHCP
Snooping, ARP Inspection, IP Source Guard so you can really enforce
the use of DHCP.