Hi, I know how to reserve a MAC address to an IP, all good there 
I am just getting mixed up if I want to unreserve is the let it go back to DHCP.
Disable or Remove? (see attached) seems to disappear it altogether and only a reboot of the router brings that device back on this list.
When the Lease expires the IP (not the MAC) will be free to be offered to another machine again…
Supposing the host is not active, otherwise will be renewed…
Remove I believe. The desktop won’t ask for a new IP till the lease expires. So yes it will disappear from the list. Once the lease expires it will ask for a new address and you’ll see it back in the list. Just because you killed it from the DHCP Server list doesn’t mean that gets broadcasted to the client. The desktop will still have the old IP. There might be a change that it might give out the address so some other device and cause a conflict. Not sure how smart the DHCP server is here.
(Saw the other reply as I tried to post but didn’t want to lose this wall of text)
The desktop won’t ask for a new IP till the lease expires
That is wrong… The host will renew the IP at 50% of the lease time, that is the Renewal Timer and the Timer will be reset upon a succesfull renewal…
We also have the Rebinding Timer, which comes in play in case there is no successfull renewal in the previous step…
That second Timer starts at 87.5% of the Lease Time…
Anyways, those are details… The actual point is that the Lease under normal circumstances will never expire as long as the host is up and active and the renewal process is always sucessfull…
Interesting read
Thanks for the answer that I click remove to unreserve.
If you made DHCP lease reservation and you want host to start using dynamic lease, then click “remove” of lease. According to explanations in previous posts host will continue to use old IP address until it tries to renew the lease. At this moment DHCP server will either allow to further use same IP (if that’s permissible according to DHCP server configuration) or will assign another IP address (again according to DHCP server configuration).