HTML 4.0 Sourcebook
(Publisher: John Wiley & Sons, Inc.)
Author(s): Ian S. Graham
ISBN: 0471257249
Publication Date: 04/01/98

Castanet URLs (Netscape Navigator 4 Only)

Castanet URLs were apparently intended as a reference to Marimba Castanet resources—I use the word “apparently,” as little documentation on this URL scheme is available. Netscape Navigator 4, the only browser to understand this scheme, treats castanet URLs as equivalent to http URLs. Thus, the references

castanet://www.utoronto.ca
<http://www.utoronto.ca>

are equivalent. Authors should thus avoid castanet URLs, since they are not understood by other browsers and http URLs provide exactly the same functionality.

Cid URLs

Cid URLs refer to a specific part of a MIME-encoded multipart message. In particular, Cid URLs are designed for use within multipart HTML (M-HTML; see Appendix B on the companion Web site) messages—such messages contain HTML documents and images, all within the same message body. The general form for a cid URL is

cid:content-id

where content-id is the URL-encoded versions of the MIME Content-ID for the desired part of the message. The format for Content-IDs in a multipart message is defined in the MIME specifications, listed at the end of Appendix B on the companion Web site. As an example, a typical ID might be:

part3-12%d7f4@flopsy.org

which identifies the machine from which the message was sent (flopsy.org) and contains a generated string (part3-12%d7f4) chosen to ensure that all IDs in the message are unique. The corresponding cid URL would be

cid:part3-12%25d7f4@flopsy.org

where the percent character in the Content-ID has been URL-encoded.

Cid URLs permit links between parts of a multipart message. For example, a single message could contain multiple HTML documents, with links between them or could contain an HTML document along with the associated images files. In this case, the images would be referenced from the document using IMG elements of the form

<IMG SRC=“cid: content-id”>

where content-id is the Content-ID for the image.

Cid URLs are supported by the Netscape Communicator mail, news, and HTML authoring clients, but only within the mail or news viewing client (Netscape Messenger); they are not understood by the Navigator 4 browser. Indeed, if a user clicks on a hypertext link referencing a cid URL in the Netscape Messenger mail client the resulting string displayed in the Navigator “Location” window is actually a mailbox URL. Cid URLs are not supported by Internet Explorer 4.