Chapter 10 Install and System Administration for FortiOS 5.0 : Advanced concepts : DHCP servers and relays : Exclude addresses in DHCP a range
  
Exclude addresses in DHCP a range
If you have a large address range for the DHCP server, you can block a range of addresses that will not be included in the available addresses for the connecting users. To do this, go to the CLI and enter the commands:
config system dhcp server
edit <server_entry_number>
config exclude-range
edit <sequence_number>
set start-ip <address>
set end-ip <address>
end
end
end
See Also
DHCP Server configuration
Service
DHCP Monitor
Breaking a address lease
DHCP servers and relays