As of now you must be wondering, how to use these awesome features that OpenStack Networking has given to us. Use Case: Single Flat Network In the simplest use case, a single OpenStack Networking network exists. This is a "shared" network, meaning it is visible to all tenants via the OpenStack Networking 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 OpenStack Compute. Floating IPs are not supported. It is common that such an OpenStack Networking 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.

Use Case: Multiple Flat Network This use case is very similar to the above Single Flat Network use case, except that tenants see multiple shared networks via the OpenStack Networking API and can choose which network (or networks) to plug into.

Use Case: Mixed Flat and Private Network This use case is an extension of the above flat network use cases, in which tenants also optionally have access to private per-tenant networks. In addition to seeing one or more shared networks via the OpenStack Networking API, tenants can create additional networks that are only visible to users of that tenant. When creating VMs, those VMs can have NICs on any of the shared networks and/or any of the private networks belonging to the tenant. This enables the creation of "multi-tier" topologies using VMs with multiple NICs. It also supports a model where a VM acting as a gateway can provide services such as routing, NAT, or load balancing.

Use Case: Provider Router with Private Networks This use provides each tenant with one or more private networks, which connect to the outside world via an OpenStack Networking router. The case where each tenant gets exactly one network in this form maps to the same logical topology as the VlanManager in OpenStack Compute (of course, OpenStack Networking doesn't require VLANs). Using the OpenStack Networking API, the tenant would only see a network for each private network assigned to that tenant. The router object in the API is created and owned by the cloud admin. This model supports giving VMs public addresses using "floating IPs", in which the router maps public addresses from the external network to fixed IPs on private networks. Hosts without floating IPs can still create outbound connections to the external network, as the provider router performs SNAT to the router's external IP. The IP address of the physical router is used as the gateway_ip of the external network subnet, so the provider has a default router for Internet traffic. The router provides L3 connectivity between private networks, meaning that different tenants can reach each others instances unless additional filtering (e.g., security groups) is used. Because there is only a single router, tenant networks cannot use overlapping IPs. Thus, it is likely that the admin would create the private networks on behalf of tenants.

Use Case: Per-tenant Routers with Private Networks A more advanced router scenario in which each tenant gets at least one router, and potentially has access to the OpenStack Networking API to create additional routers. The tenant can create their own networks, potentially uplinking those networks to a router. This model enables tenant-defined multi-tier applications, with each tier being a separate network behind the router. Since there are multiple routers, tenant subnets can be overlapping without conflicting, since access to external networks all happens via SNAT or Floating IPs. Each router uplink and floating IP is allocated from the external network subnet.