A directory entry can be a hard link , in which the i-number points directly to another file. A hard link to a file is indistinguishable from the file itself. When a hard link is made, then the i-numbers of two different directory file entries point to the same inode. For that reason, hard links cannot span across file systems. A soft link is implemented as a directory file entry containing a pathname.
Soft links are distinguishable from files, and can span across file systems.
Not all versions of UNIX support soft links. When we speak of a UNIX file system, we are actually referring to an area of physical memory represented by a single i-list. A UNIX machine may be connected to several file systems, each with its own i-list. One of those i-lists points to a special storage area, known as the root file system . The root file system contains the files for the operating system itself, and must be available at all times.