neutron-lib/api-ref/source/v2/samples/subnets/subnet-create-request.json

{
    "subnet": {
        "network_id": "d32019d3-bc6e-4319-9c1d-6722fc136a22",
        "ip_version": 4,
        "cidr": "192.168.199.0/24"
    }
}