While having the ability to create your own networks is a huge leap forward, it means nothing without a means to connect containers to it. In previous versions of Docker, this was traditionally done during container runtime by passing the --net flag specifying which network the container should use. While this is certainly still the case, the docker network subcommand also allows you to connect and disconnect running containers to existing networks.
The docker network subcommand was introduced in Docker 1.9, so you'll need a Docker host running at least that version. In our examples, we'll be using Docker version 1.12. You'll also want to have a good understanding of your current network layout, so you can follow along as we inspect the current configuration. It is assumed that each Docker host is in its native configuration.