Web design development. Everything about making hyper text hyper

The <A> tag actually serves two purposes: Creating a link to another resource on the Web, and creating a fragment identifier within a document.

The basics: Creating a link

When used to create a link, the <A> tag never appears without the HREF attribute. This attribute supplies the URL of the resource to which the link refers. The content of the tag (i.e., everything between the <A> and </A> tags) is displayed differently, telling the user that it is a link. Usually, it is presented in a different color, with an underline, or (in the case of images) with a colored border.

A typical <A> tag might look like this:

<A HREF="http://finalhost.com"><B>Final Host</B></A>

The target of this link is the URL of finalhost. Within the document, the words "Final Host" will be displayed as a link, like this: Final Host.

To be honest, there is little more to say about using the <A> tag to create links. However, there is much more to say about how you create those links.

Relative and absolute links

Technically, every URL has two parts: The scheme and the scheme-specific part. URLs that point to other documents on the Web use the http scheme; other common URLs use the ftp, mailto, and nntp schemes. Each has a unique scheme-specific part that provides all the details the browser needs to access that resource.

The http URL requires you to provide several bits of information, including a server name, a document directory, and a document name. Most, if not all, of these components are optional, and there are rules that let the browser fill in the missing pieces if you don't provide them.

If you do provide all the pieces, the URL is said to be absolute: It absolutely defines where the resource can be found on the Web. If one or more pieces are missing, the URL is said to be relative: The missing pieces are copied from the document containing the <A> tag. This makes the value of the URL dependent upon, and relative to, the location of the containing document. Here are some samples:

http://finalhost.com/soundadvice/
buildingblocks.html

//finalhost.com
http:buildingblocks.html
/soundadvice

Only the first URL is absolute: It completely specifies the scheme, the server, the directory, and the document to be retrieved. In fact, it is the URL of this document. The second URL is relative, because it is missing the scheme. The browser will use the scheme of the owning document. The third URL is relative, since it is missing a server name and a document directory. Again, the browser will use the server name and document directory of the containing document to fill in the missing pieces. The last URL is missing the scheme and server, but supplies a document directory. Again, the browser picks up the containing document's scheme and server name.

Which should you use?

Both kinds of URLs are useful. In general, you should use absolute URLs when referencing resources that are on a server different from the current document's, or whose server is not likely to change.

Relative URLs are much more useful. You should use relative URLs when referencing documents on the same server, in the same collection, or within the same directory structure. A good rule is to eliminate as many elements of the URL as you can and still have it work correctly.

Why is this? An important feature of Web-based documents that is often overlooked is portability. If you use relative links throughout your documents, you can move these documents to a different directory, a different server, or even a different scheme without ever changing the internal links.

Consider a collection of related documents that link to one another. If you place all of these documents in the same directory on your server, you can create links using only the document names, removing the scheme, server name, and document directory. If you choose to move the collection to a different directory, the links will work correctly, inheriting the directory from the first document in the collection you access. If you get more ambitious and move the whole directory to a different server, the links will still work, inheriting the server name in a similar manner. If you decide to deliver a static version of this directory via CD (changing the scheme to "file"), the links will still work.

Relative URLs take a little extra work, creating the right document collection architecture, but pay off in reduced maintenance costs and ease of portability.

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