The simplest use case is a single network. This is a "shared" network, meaning it is visible to all tenants via the Networking API. Tenant VMs have a single NIC, and receive a fixed IP address from the subnet(s) associated with that network. This use case essentially maps to the FlatManager and FlatDHCPManager models provided by Compute. Floating IPs are not supported.
This network type is often created by the OpenStack administrator to map directly to an existing physical network in the data center (called a "provider network"). 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.
