On Windows Server 2008 R2, you cannot add a reservation using the DHCP MMC or netsh commands if the reservation is outside of the distribution range of the DHCP scope, even if it falls in the subnet defined by the subnet mask of the scope.
Distribution Range: 10.10.1.21-10.10.1.230
Subnet Mask: 255.255.254.0
Adding the IP address 10.10.0.164 will fail. The message that will be displayed is: The specified DHCP client is not a reserved client
When adding a new reservation, the DHCP server checks if the IP address is within the defined distribution range of the scope.
This is by design. If the DHCP scope range does not cover the whole subnet defined by the subnet mask, you cannot add a reservation outside of the configured range. You can, however, extend the distribution range so that the whole subnet is covered, and specify the reservation after this step. If you do not want to distribute addresses from the whole subnet, you can also define an exclusion range. A reservation which falls into an exclusion range will still work.
Adding a reservation which is outside of the distribution range of a scope was possible in Windows Server 2003 and Windows Server 2008.
However, this is considered to be an invalid configuration and as such, the behavior has been changed so that it is not possible any more to create such a reservation.