Technical Tip: How to configure unlimited DHCP lease only for reserved IP addresses on FortiGate

To configure different DHCP lease durations based on whether an IP is reserved or dynamically assigned:

1. Configure the DHCP server with two separate IP ranges:

• A small range (for example, 10 IP addresses) will be used for reserved IP addresses (MAC-IP bindings).

• A larger range will be used for dynamically assigned IP addresses.

• Do not configure a lease-time on the reserved range to make it unlimited.

• Explicitly configure the lease-time (in seconds) for the dynamic range.

2. Configure the MAC-IP bindings to use IP addresses from the reserved range.

Example Configuration:

config system dhcp server
    edit 1
        set interface "port3"
        set default-gateway 172.31.205.234
        set netmask 255.255.240.0

            config ip-range
                edit 1
                    set start-ip 172.31.192.1
                    set end-ip 172.31.192.10
                next
                edit 2
                    set start-ip 172.31.192.11
                    set end-ip 172.31.192.254
                    set lease-time 3600
                next
            end
    next
end

Note:

Separate DHCP lease times for distinct IP ranges can be configured only via CLI

3. Add MAC-IP reservations: Use IPs only from the reserved pool (172.31.192.1 to 172.31.192.10 in this example).

config system dhcp reserved-address
    edit 1
        set mac 00:11:22:33:44:55
        set ip 172.31.192.2
        set interface "port3"
    next
    edit 2
        set mac 00:11:22:33:44:66
        set ip 172.31.192.3
        set interface "port3"
    next
end

Verify the DHCP lease duration:

execute dhcp lease-list