[neutron][ipam] how is the first subnet determined by neutron?

Hi, When there are multiple subnets in a provider network, FIP is always allocated from the "first" subnet. What I see is that, the "first" subnet is the subnet added as the first. Or the first subnet from "openstack network show". Is that how subnets ordered, by creation order? Is there any way to change this order by manipulating, like subnets table in DB? I have two subnets, I want to allocate FIP from the second subnet, instead of the first, without clean them up and recreate them. Is that possible? Note, not allocate specific address, but auto allocation. Thanks! Tony