Using this recipe will result in the DHCP server assigning an IP address to every client requesting one. This may not be what we want. Checking the Deny unknown clients checkbox will cause the DHCP server to assign IP addresses only to clients whose MAC addresses are listed under DHCP Static Mappings for this Interface. DHCP requests from all other clients will then be ignored:

This differs from the Enable Static ARP entries option. Checking this option will result in unknown clients being assigned IP addresses even if Deny unknown clients is enabled (for that matter, they will be assigned an IP address even if the DHCP server is disabled), but only clients listed under DHCP Static Mappings for this Interface will be allowed to communicate with the firewall.