Some companies use isolated networks or no network at all for security purposes, but it is still important to use version control tools like Git. Git can be used without a remote repository on a single machine, but when working with multiple machines, a remote repository can be created on a USB memory stick or portable hard drive. In locked down development environments where memory sticks are blocked, Git can still be used by copying the local repository directory to another computer via CD or other media. Another option is to use git bundle, which allows for part or all of a repository to be compressed into a single file that can be cloned and fetched from. Creating a local remote repository on a designated “server” machine can also help synchronize Git repositories across multiple computers. While these methods may not be as convenient as using a central server like GitHub, they offer alternatives for working without a network and avoiding versioning issues.
https://www.gibbard.me/using_git_offline/