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. We like to recognize contributors by citing them in our release notes. If you wish to be thanked in this manner please respond with how you want to be identified. Most often it is by name and/or organization. Thanks for taking the time to report this issue. Regards, Thomas Markwalder ISC Software Engineering