6.2.1.1. Handling reserved and unsafe characters

http://www.calculator.com/compute?3%2f4

Table 6-1. Reserved and Unsafe Characters and Their URL Encodings

In general, you should always encode a character if there is some doubt as to whether it can be placed as-is in a URL. As a rule of thumb, any character other than a letter, number, or any of the characters $-_.+!*'( ) should be encoded.

It is never an error to encode a character, unless that character has a specific meaning in the URL. For example, encoding the slashes in an http URL causes them to be used as regular characters, not as pathname delimiters, breaking the URL.

6.2.2.1. The http server

It has become something of a convention that webmasters name their servers www for quick and easy identification on the Web. For instance, O'Reilly & Associates' web server's name is www, which, along with the publisher's domain name, becomes the very easily remembered web site www.oreilly.com. Similarly, Sun Microsystems' web server is named www.sun.com; Apple Computer's is www.apple.com, and even Microsoft makes their web server easily memorable as www.microsoft.com. The naming convention has very obvious benefits, which you, too, should take advantage of if you are called upon to create a web server for your organization.

It'd be a dull diversion to tell you now what the numbers mean or how to derive an IP address from a domain name, particularly since you'll rarely if ever use one in a URL. Rather, this is a good place to hyperlink: pick up any good Internet networking treatise for rigorous detail on IP addressing, such as Ed Krol's The Whole Internet User's Guide and Catalog (O'Reilly & Associates).

6.2.2.2. The http port

The default URL port for web servers is 80. Special secure web servers (Secure HTTP, SHTTP or Secure Socket Layer, SSL) run on port 443. Most web servers today use port 80; you need to include a port number along with an immediately preceding colon in your URL if the target server does not use port 80 for web communication.

When the Web was in its infancy, pioneer webmasters ran their Wild Wild Web connections on all sorts of port numbers. For technical and security reasons, system-administrator privileges are required to install a server on port 80. Lacking such privileges, these webmasters chose other, more easily accessible, port numbers.

Now that web servers have become acceptable and are under the care and feeding of responsible administrators, documents being served on some port other than 80 or 443 should make you wonder if that server is really on the up and up. Most likely, the maverick server is being run by a clever user unbeknownst to the server's bona fide system administrators.

6.2.2.3. The http path

It has become a convention that for easy identification, HTML document names end with the suffix .html (otherwise they're plain ASCII text files, remember?). Although recent versions of Windows allow longer suffixes, their users often stick to the three-letter .htm name suffix for HTML documents.

Different servers have different ways of locating documents within a user's home directory. Many search for the documents in a directory named public_html. Unix-based servers are fond of the name index.html for home pages. When all else fails, servers tend to cough up the first text document in the home page directory.

6.2.2.4. The http document fragment

The fragment name and the preceding hash symbol are optional; omit them when referencing a document without defined fragments.

Formally, the fragment element only applies to HTML or XHTML documents. If the target of the URL is some other document type, the fragment name may be misinterpreted by the browser.

Fragments are useful for long documents. By identifying key sections of your document with a fragment name, you make it easy for readers to link directly to that portion of the document, avoiding the tedium of scrolling or searching through the document to get to the section that interests them.

As a rule of thumb, we recommend that every section header in your documents be accompanied by an equivalent fragment name. By consistently following this rule, you'll make it possible for readers to jump to any section in any of your documents. Fragments also make it easier to build tables of contents for your document families.

6.2.2.5. The http search parameter

6.2.2.6. Sample http URLs

Here are some sample http URLs:

http://www.oreilly.com/catalog.html
http://www.oreilly.com/
http://www.kumquat.com:8080/
http://www.kumquat.com/planting/guide.html#soil_prep
http://www.kumquat.com/find_a_quat?state=Florida

The first example is an explicit reference to a bona fide HTML document named catalog.html that is stored in the root directory of the www.oreilly.com server. The second references the top-level home page on that same server. That home page may or may not be catalog.html. Sample three, also, assumes that there is a home page in the root directory of the www.kumquat.com server, and that the web connection is to the nonstandard port 8080.

The fourth example is the URL for retrieving the web document named guide.html from the planting directory on the www.kumquat.com server. Once retrieved, the browser should display the document beginning at the fragment named soil_ prep.

6.2.3.1. The javascript URL arguments

6.2.4.1. The ftp user and password

If you are using an ftp URL to access a site that requires a username and password, include the user and password components in the URL, along with the colon (:) and "at" sign (@). More commonly, you'll be accessing an anonymous FTP server, and the user and password components can be omitted.

If you keep the user component along with the "at" sign, but omit the password and the preceding colon, most browsers will prompt you for a password after connecting to the FTP server. This is the recommended way of accessing authenticated resources on an FTP server; it prevents others from seeing your password.

We recommend you never place an ftp URL with a username and password in any HTML document. The reasoning is simple: anyone can retrieve the document, extract the username and password from the URL, log into the FTP server, and tamper with its documents.

6.2.4.2. The ftp server and port

If the port and its preceding colon are omitted, the default port of 21 is used. It is necessary to specify the port only if the FTP server is running on some port other than 21.

6.2.4.3. The ftp path and transfer type

If the typecode is set to d, the path is assumed to be a directory. The browser will request a listing of the directory contents from the server and display this listing to the user. If the typecode is any other letter, it is used as a parameter to the FTP type command before retrieving the file referenced by the path. While some FTP servers may implement other codes, most servers accept i to initiate a binary transfer and a to treat the file as a stream of ASCII text.

6.2.5.1. The file server

If the server is omitted, or the special name localhost is used, the file is assumed to reside on the same machine upon which the browser is running. In this case, the browser simply accesses the file using the normal facilities of the local operating system. In fact, this is the most common usage of the file URL. By creating document families on a diskette or CD-ROM and referencing your hyperlinks using the file://localhost/ URL, you create a distributable, standalone document collection that does not require a network connection to use.

6.2.6.2. Accessing single messages

unique_string@server

The unique_string is a sequence of ASCII characters; the server is usually the name of the machine from which the message originated. The unique_string must be unique among all the messages that originated from the server. A sample URL to access a single message might be:

news:12A7789B@news.kumquat.com

6.2.7.1. The nntp server and port

If the port and its preceding colon are omitted, the default port of 119 is used.

6.2.7.2. The nntp newsgroup and article

6.2.7.3. Sample nntp URLs

A sample nntp URL might be:

nntp://news.kumquat.com/alt.fan.kumquats/417

6.2.8.1. Defining mail header fields

6.2.9.1. The telnet user and password

Just like the ftp URL, if you omit the password from the URL, the browser should prompt you for a password just before contacting the telnet server.

If you omit both the user and password, the telnet occurs without supplying a user name. For some servers, telnet automatically connects to a default service when no username is supplied. For others, the browser may prompt for a username and password when making the connection to the telnet server.

6.2.9.2. The telnet server and port

6.2.10.1. The gopher server and port

If the port and its preceding colon are omitted, the default port of 70 is used.

6.2.10.2. The gopher path

The path can take one of three forms:

type/selector
type/selector%09search
type/selector%09search%09gopherplus

If the Gopher resource is actually a Gopher search engine, the search component provides the string for which to search. The search string must be preceded by an encoded horizontal tab (%09).

If the Gopher server supports Gopher+ resources, the gopherplus component supplies the necessary information to locate that resource. The exact content of this component varies based upon the resources on the gopher server. This component is preceded by an encoded horizontal tab (%09). If you want to include the gopherplus component but omit the search component, you must still supply both encoded tabs within the URL.

6.2.11.1. Relative schemes and servers

another-doc.html

is equivalent to the absolute URL:

http://www.kumquat.com/another-doc.html

Table 6-2. Forming an Absolute URL

6.2.11.2. Relative document directories

Another common form of a relative URL omits the leading slash and one or more directory names from the beginning of the document pathname. The directory of the base URL is automatically assumed to replace these missing components. It's the most common abbreviation, because most authors place their collection of documents and subdirectories of support resources in the same directory path as the home page. For example, you might have a special/ subdirectory containing FTP files referenced in your document. Let's say that the absolute URL for that document is:

http://www.kumquat.com/planting/guide.html

A relative URL for the file README.txt in the special/ subdirectory, looks like this:

ftp:special/README.txt

You'll actually be retrieving:

ftp://www.kumquat.com/planting/special/README.txt

Table 6-3. Forming an Absolute FTP URL

Common "dot-slash" pathname notations also let you express the current directory ("./") and directory above the current directory (parent; "../") in a relative URL. The current directory notation is rarely used, since it is redundant. But the parent notation lets you set the target URL to directories in other branches of the filesystem hierarchy.

For example, if the directory portion of the current URL is /planting/special/, and you want to reference an HTML document named new_ground.html in planting/standard/, you may simply form the relative URL as:

../standard/new_ground.html

You'll actually be retrieving:

http://www.kumquat.com/planting/standard/new_ground.html

Note that parent notation has limits. For instance, most web servers will not let you navigate above the base directory: http://www.kumquat.com/../ probably won't deliver any document or directory listing to your browser.

6.2.11.3. Using relative URLs