Like Nova Networking, Neutron manages software-defined networking for your OpenStack installation. However, unlike Nova Networking, you can configure Neutron for advanced virtual network topologies, such as per-tenant private networks and more.
Neutron has the following object abstractions: networks, subnets, and routers. Each has functionality that mimics its physical counterpart: networks contain subnets, and routers route traffic between different subnet and networks.
Any given Neutron set up has at least one external network. This network, unlike the other networks, is not merely a virtually defined network. Instead, it represents the view into a slice of the external network that is accessible outside the OpenStack installation. IP addresses on the Neutron external network are accessible by anybody physically on the outside network. Because this network merely represents a slice of the outside network, DHCP is disabled on this network.
In addition to external networks, any Neutron set up has one or more internal networks. These software-defined networks connect directly to the VMs. Only the VMs on any given internal network, or those on subnets connected through interfaces to a similar router, can access VMs connected to that network directly.
For the outside network to access VMs, and vice versa, routers between the networks are needed. Each router has one gateway that is connected to a network and many interfaces that are connected to subnets. Like a physical router, subnets can access machines on other subnets that are connected to the same router, and machines can access the outside network through the gateway for the router.
Additionally, you can allocate IP addresses on an external networks to ports on the internal network. Whenever something is connected to a subnet, that connection is called a port. You can associate external network IP addresses with ports to VMs. This way, entities on the outside network can access VMs.
Neutron also supports security groups. Security groups enable administrators to define firewall rules in groups. A VM can belong to one or more security groups, and Neutron applies the rules in those security groups to block or unblock ports, port ranges, or traffic types for that VM.
Each plug-in that Neutron uses has its own concepts. While not vital to operating Neutron, understanding these concepts can help you set up Neutron. All Neutron installations use a core plug-in and a security group plug-in (or just the No-Op security group plug-in). Additionally, Firewall-as-a-service (FWaaS) and Load-balancing-as-a-service (LBaaS) plug-ins are available.
