Hypertext Linking

Anchors have a standard format that allows any Web browser to interpret a link and perform the proper function (called a method) for that type of link. Links can refer to other documents, specific locations within the same document, or can perform operations, such as retrieving a file using FTP for display by the browser. URLs can refer to a specific location by an absolute pathname, or can be relative to the current document, which is more convenient when managing a large site.

Note: You can use hypertext links to navigate through a document or to move from document to document. However, HTML does not support returning you to the anchor point of a link within a document. If you use a hypertext link within a document, and then use the Back button, you do not return to the anchor, but to the previous point that you reached through a link.

What is a URL?

The general format of a URL is:

method://machine-name/path/foo.html

The following example would fetch the document index.html from the server www.acme.com using the HTTP protocol.

http://www.acme.com/index.html

A Uniform Resource Locator (URL) has the following format:

method://servername:port/pathname#anchor

The components of the URL are as follows:

method

is the name of the operation that is performed to interpret this URL. The most common methods are:

file

Read a file from the local disk. The filename is interpreted on the user's machine, so this can be used to display any file that resides on the user's disk.

For example: file:/home/jjones/jjones.html displays the file jjones.html from the directory /home/jjones on the local machine.

http

Access a page over the network by way of the HTTP protocol. (This is the most common method, usually used to get an HTML document.)

For example: http://www.acme.com/
accesses Acme's home page.

mailto

Activate a mail session to the specified username and host.

For example: mailto:jjones@us.acme.com mails a message to jjones if the browser supports mail creation. Note that the mailto method does not require double forward slashes after the colon.

ftp

Retrieve a file using anonymous FTP from a server.

For example, ftp://hostname/directory/filename

servername

is optional and indicates the full hostname of a machine connected to the network. For example, www.oracle.com is the fully qualified hostname of Oracle's web server. If a servername is not specified, the URL is a relative link, and it is assumed that the file is on the same server that was used to display the current page. An IP address may be used instead of a hostname, although it is not recommended to build content with embedded IP addresses.

port

is the TCP port number that the web server is running on. The default is 80 if port is not specified. This parameter is not used in most URLs.

pathname

is the relative or absolute pathname of the document being accessed by this URL. Web servers can be configured to interpret certain pathnames differently. For instance, CGI applications work by configuring the HTTP server to recognize that files within certain directories should be executed instead of being returned to the browser.

For example: http://www.acme.com/index.html

In this example, an HTTP connection is made to index.html, which is the name of the file to be accessed on the server www.acme.com using port 80 (the default). The file could have a full UNIX-style pathname to indicate a document contained in a lower level directory. If there is no directory component in the pathname, the document must be located in the server's document root directory which is configured by the server administrator. If the pathname part of the URL is missing, many servers provide a directory listing of the document root directory or access a specific `top level' file (usually index.html).

#anchor

The named anchor points to a specific location within an HTML page. In addition to specifying a document name, specifying #anchor will cause most browsers to move the top of the display to the point referred to by the anchor. These anchor names are inserted into documents with the NAME tag as explained under "Linking to Sections on Pages" later in this appendix.

Structure of an Anchor Link within a Document

<A HREF="URL">text_to_be_displayed_highlighted</A>

The <A HREF="URL"> tag opens the anchor link and the </A> tag closes it. All the text between those tags is displayed highlighted in some way by a web browser. A common technique is to display it underlined and in blue or some other user-selected contrasting color.

The URL part of the tag refers to the text of the URL reference as defined in the previous section. The URL text does not appear on the user's screen; it is only used when the user activates the link, usually by clicking on it with the mouse.

An example HTML segment:

For interesting products see
<A HREF="http://www.acme.com">Acme's home page.</A>

This line would produce on the user's screen:

For interesting products see Acme's home page.

Graphics within HTML Documents

There are two ways to use graphics from within an HTML document. The first is by embedding them within the document itself, so the user's screen will display the graphics within the context of the other elements of your document (such as explanatory text). This is the most common technique used by HTML designers and is called an "inline image". The syntax for specifying this is:

<IMG SRC="URL" ALT="text" 
   ALIGN=["top"|"middle"|"bottom"|"texttop"] ISMAP>

The elements in this syntax statement are as follows:

URL

ALT="text"

ALIGN

ISMAP

For example, the following line will cause the file logo.gif to be fetched from server www.oracle.com and the text Oracle Logo to be displayed if the user has graphic display turned off.

<IMG SRC="http://www.oracle.com/logo.gif" ALT="Oracle Logo">

Linking to Sections on Pages

To create a link, follow these steps:

1. Create a named anchor marker in each section title that you might want to jump to. The HTML syntax is as follows:

   	<A NAME="named_anchor"> Text_to_link_to</A>

2. In the following example, a hidden reference marker is placed in a head2 level:

   	<H2><A NAME="intro">Introduction to Hockey</A></H2>

3. Create the link by entering:

   	<A HREF="#named_anchor">Text</A>

4. For example, the link might appear as:

   	<A HREF="#intro">Gordie Howe</A>

When the user clicks on "Gordie Howe," the browser displays the heading "Introduction to Hockey" at the top of the screen.

Note: As specified earlier in the URL syntax, a link to a section can appear in the same document or in another document. The example here describes a link to a specific section of the same document.

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