Creates a subnet group. A subnet group is a collection of subnets (typically private) that you can designate for your clusters running in an Amazon Virtual Private Cloud (VPC) environment. When you create a cluster in an Amazon VPC, you must specify a subnet group. MemoryDB uses that subnet group to choose a subnet and IP addresses within that subnet to associate with your nodes. For more information, see <a href="https://docs.aws.amazon.com/MemoryDB/latest/devguide/subnetgroups.html">Subnets and subnet groups</a>.