Chapter 10 Install and System Administration for FortiOS 5.0 : Advanced concepts : DHCP servers and relays : DHCP Server configuration
  
DHCP Server configuration
To add a DHCP server, go to System > Network > Interface. Edit the interface, and select Enable for the DHCP Server row.
DHCP Server IP
This appears only when Mode is Relay. Enter the IP address of the DHCP server where the FortiGate unit obtains the requested IP address.
Address Range
By default, the FortiGate unit assigns an address range based on the address of the interface for the complete scope of the address. For example, if the interface address is 172.20.120.230, the default range created is 172.20.120.231 to 172.20.120.254. Select the range and select Edit to adjust the range as needed, or select Create New to add a different range.
Netmask
Enter the netmask of the addresses that the DHCP server assigns.
Default Gateway
Select to either use the same IP as the interface or select Specify and enter the IP address of the default gateway that the DHCP server assigns to DHCP clients.
DNS Server
Select to use the system’s DNS settings or select Specify and enter the IP address of the DNS server.
Advanced
Mode
Select the type of DHCP server the FortiGate unit will be. By default, it is a server. Select Relay if needed. When Relay is selected, the above configuration is replaced by a field to enter the DHCP Server IP address.
Type
Select to use the DHCP in regular or IPsec mode.
MAC Address Access Control List
Select to match an IP address from the DHCP server to a specific client or device using its MAC address.
In a typical situation, an IP address is assigned ad hoc to a client, and that assignment times out after a specific time of inactivity from the client, known as the lease time. To ensure a client or device always has the same IP address, that is, there is no lease time, use IP reservation.
Add from DHCP Client List
If the client is currently connected and using an IP address from the DHCP server, you can select this option to select the client from the list.