In the simplest use case, a single Quantum network exists. This is a "shared" network, meaning it is visible to all tenants via the Quantum API. Tenant VMs have a single NIC, and receive a fixed IP address from the subnet(s) associated with that network. This essentially maps to the FlatManager and FlatDHCPManager models provided by Nova. Floating IPs are not supported.
It is common that such a Quantum network is a "provider network", meaning it was created by the OpenStack administrator to map directly to an existing physical network in the data center. This allows the provider to use a physical router on that data center network as the gateway for VMs to reach the outside world. For each subnet on an external network, the gateway configuration on the physical router must be manually configured outside of OpenStack.
