Excerpt from RFC 1738
3.10 FILES
The file URL scheme is used to designate files accessible on a
particular host computer. This scheme, unlike most other URL schemes,
does not designate a resource that is universally accessible over the
Internet.
A file URL takes the form:
file:///
where is the fully qualified domain name of the system on
which the is accessible, and is a hierarchical
directory path of the form //.../.
For example, a VMS file
DISK$USER:[MY.NOTES]NOTE123456.TXT
might become
As a special case, can be the string "localhost" or the empty
string; this is interpreted as `the machine from which the URL is
being interpreted'.
The file URL scheme is unusual in that it does not specify an
Internet protocol or access method for such files; as such, its
utility in network protocols between hosts is limited.