Nebula is a "scalable overlay networking tool" that allows users to connect computers running anywhere in the world as if they were attached to the same network switch. It was created by the team at Slack to provide a way for hosts spread across a number of data centers and cloud providers to communicate securely. Within the software-defined network created by Nebula, users can place hosts into groups and create traffic routing rules that define how hosts and groups are allowed to talk to one another. In the Medium post linked from the project README, users can find a longer introduction to the project and its goals. The Technical Overview portion of the README provides links to detailed protocol specifications that describe how Nebula works. Nebula releases are provided for Windows, macOS, Linux, and FreeBSD. Clients for mobile devices are available in the iOS and Android stores.