As you can see, hard links work on the inode level to point to a particular file. On Linux systems, hard links have several limitations. For one, you can only make hard links to files, not directories. That's right; even though . and .. are system-created hard links to directories, you (not even as the "root" user) aren't allowed to create any of your own.

The second limitation of hard links is that they can't span filesystems. This means that you can't create a link from /usr/bin/bash to /bin/bash if your / and /usr directories exist on separate filesystems.