The set of letters, numbers, and permissible punctuation or symbols assigned to a file that distinguishes it from all other files in any particular hierarchy. Users save or call specific blocks of information with discrete filename "handles" or keys. Filenames, often with extensions, identify the type or purpose of data. In MS-DOS, a filename can be up to eight alphanumeric characters long, with a three character extension. On Macintosh computers, a filename can be up to 31 characters long, and can include any symbol other than the colon (:), which is used to separate the elements of a path. In the OS/2 High-Performance File System (HPFS), filenames can be up to 254 characters long, but no path can exceed 259 characters. In the Windows NT File System (NTFS), filenames can be up to 255 characters long. Although HPFS and NTFS both support spaces in filenames, web servers do not allow spaces. UNIX does not allow spaces in filenames, and is case sensitive. The UNIX and Macintosh operating systems permit a file to have more than one name, which is called "alias" or "symbolic link". See path, slash, backslash, pipe, MIME, internet address.