For clients that need to keep the same IP address, you can either manually configure a static IP address, or assign a reservation ...

For clients that need to keep the same IP address, you can either manually configure a static IP address, or assign a reservation on the DHCP server. Reservations are permanent lease assignments that are used to ensure that a specified client on a subnet can always use the same IP address. You can use DHCP reservations for hosts that require a consistent IP address, but that do not need to be statically configured.