characters. A URLs may be represented in a variety of ways: e.g., ink
   on paper, or a sequence of octets in a coded character set. The
   interpretation of a URL depends only on the identity of the
   characters used.

   In most URL schemes, the sequences of characters in different parts
   of a URL are used to represent sequences of octets used in Internet
   protocols. For example, in the ftp scheme, the host name, directory
   name and file names are such sequences of octets, represented by
   parts of the URL.  Within those parts, an octet may be represented by




 
RFC 1738            Uniform Resource Locators (URL)        December 1994


   the chararacter which has that octet as its code within the US-ASCII
   [20] coded character set.

   In addition, octets may be encoded by a character triplet consisting
   of the character "%" followed by the two hexadecimal digits (from
   "0123456789ABCDEF") which forming the hexadecimal value of the octet.
   (The characters "abcdef" may also be used in hexadecimal encodings.)

   Octets must be encoded if they have no corresponding graphic
   character within the US-ASCII coded character set, if the use of the
   corresponding character is unsafe, or if the corresponding character
   is reserved for some other interpretation within the particular URL
   scheme.

   No corresponding graphic US-ASCII:

   URLs are written only with the graphic printable characters of the
   US-ASCII coded character set. The octets 80-FF hexadecimal are not
   used in US-ASCII, and the octets 00-1F and 7F hexadecimal represent
   control characters; these must be encoded.

   Unsafe:

   Characters can be unsafe for a number of reasons.  The space
   character is unsafe because significant spaces may disappear and
   insignificant spaces may be introduced when URLs are transcribed or
   typeset or subjected to the treatment of word-processing programs.
   The characters "<" and ">" are unsafe because they are used as the
   delimiters around URLs in free text; the quote mark (""") is used to
   delimit URLs in some systems.  The character "#" is unsafe and should
   always be encoded because it is used in World Wide Web and in other
   systems to delimit a URL from a fragment/anchor identifier that might
   follow it.  The character "%" is unsafe because it is used for
   encodings of other characters.  Other characters are unsafe because
   gateways and other transport agents are known to sometimes modify
   such characters. These characters are "{", "}", "|", "\", "^", "~",
   "[", "]", and "`".

   All unsafe characters must always be encoded within a URL. For
   example, the character "#" must be encoded within URLs even in
   systems that do not normally deal with fragment or anchor
   identifiers, so that if the URL is copied into another system that
   does use them, it will not be necessary to change the URL encoding.









 
RFC 1738            Uniform Resource Locators (URL)        December 1994


   Reserved:

   Many URL schemes reserve certain characters for a special meaning:
   their appearance in the scheme-specific part of the URL has a
   designated semantics. If the character corresponding to an octet is
   reserved in a scheme, the octet must be encoded.  The characters ";",
   "/", "?", ":", "@", "=" and "&" are the characters which may be
   reserved for special meaning within a scheme. No other characters may
   be reserved within a scheme.

   Usually a URL has the same interpretation when an octet is
   represented by a character and when it encoded. However, this is not
   true for reserved characters: encoding a character reserved for a
   particular scheme may change the semantics of a URL.

   Thus, only alphanumerics, the special characters "$-_.+!*'(),", and
   reserved characters used for their reserved purposes may be used
   unencoded within a URL.

   On the other hand, characters that are not required to be encoded
   (including alphanumerics) may be encoded within the scheme-specific
   part of a URL, as long as they are not being used for a reserved
   purpose.

2.3 Hierarchical schemes and relative links