In the Neutron data model, a subnet is an IPv4 or IPv6 address block from which IP addresses can be assigned to virtual machine instances and other network resources. Each subnet must have a subnet mask represented by a Classless Inter-Domain Routing (CIDR) and must be associated with a network, as seen here:
In the preceding diagram, three isolated VLAN networks each have a corresponding subnet. Instances and other devices cannot be attached to networks without an associated subnet. Instances connected to a network can communicate among one another, but they are unable to connect to other networks or subnets without the use of a router. More information on routers can be found in Chapter 6, Routing. The following diagram shows how a Neutron subnet describes various Layer 3 attributes in the OSI model:
When creating subnets, users can specify IP allocation pools that limit which addresses in the subnet are available for allocation. Users can also define a custom gateway address, a...