Hello Tim:

You'll be pleased to hear that we've addressed your issue in our next release of ISC DHCP, 4.4.0, which is due out first quarter 2018. 

As you reported, the server would return a client's previously released prefix lease even when the client subsequently provided a prefix length hint that did not match that lease.  Now the server will only return the previous lease if it exactly matches the hint or the prefix-length-mode is set to "ignore".  Otherwise, it will attempt to allocate a new prefix based on the hint and the prefix-length-mode.

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Thanks for taking the time to report this issue. 

Regards,

Thomas Markwalder
ISC Software Engineering