The network ID of the subnet is the neutron_network_id in the subnet fields in Subnet > Creating a Subnet in the Virtual Private Cloud API Reference.
Parameter neutron_network_id indicates the network ID (native OpenStack API). This uniquely identifies a subnet on the management console.
The subnet ID of the subnet is the neutron_subnet_id in the subnet fields in Subnet > Creating a Subnet in the Virtual Private Cloud API Reference.
Parameter neutron_subnet_id indicates the subnet ID (native OpenStack API).

What Are the Differences Between the Network ID and Subnet ID of a Subnet?