you can modify an existing subnet; however, you should be aware that doing so can cause problems. if you modify the subnet and then reboot the client, the firebox may return an ip address that does not work with certain devices or services. from policy manager:

    1 select network =] dhcp server.

    2 click the subnet to review or modify. click edit.

    3 the dhcp subnet properties dialog box appears.

    4 when you have finished reviewing or modifying the subnet, click ok.