D-Link DIR-68 User Manual page 35

Section  - Configuration
Note:..The Revoke option will not disconnect a PC with a current network session from the network; you would need to use MAC Address
Filter to do that. Revoke will only free up a DHCP Address for the very next requester. If the previous owner is still available, those
two devices may both receive an IP Address Conflict error, or the second device may still not receive an IP Address; in that case,
you may still need to extend the "DHCP IP Address Range" to address the issue, it is located in the DHCP Server section.
Reserve:
The Reserve option converts this dynamic IP allocation into a DHCP Reservation and adds the corresponding entry to the DHCP Reservations
List.
D-Link DIR-68 User Manual 5