Anatomy of an http URL | WebReference

Anatomy of an http URL

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Anatomy of an http URL

The most widely used URL scheme is the http scheme. The http URL scheme is used to locate documents that reside on Web servers.

A Web server is more accurately called an HTTP server. HTTP stands for Hypertext Transfer Protocol, and is a protocol designed to transfer hypertext documents over the Internet. It is used to transfer almost all of the documents you download using your Web browser. Knowing a bit about HTTP may be useful for HTML authors, but we won't cover any of it right now.

An http URL may be broken down as shown below:

http://WebReference.com:80/html/tutorial2/2.html?query
|--|   |--------------||-||--------------------||----|
 1      2               3  4                     5

The first part, http, is the scheme name, which I explained previously. It is followed by a colon (:) and two slash characters (//).

After that follows the hostname of the computer on which the document resides. You probably already know what a hostname is; but just in case you don't, here's a few words on the topic:

Computers on the Internet have a numeric address, called an IP address. This is a set of four numbers ranging from 0 to 255. For example, the IP address of the computer on which WebReference.com's HTTP server is running is:

199.35.192.185

This address acts much like a phone number. If you "dial" this address into your computer's Internet software, it will find out where the computer is and figure out a way to get to it.

The problem with IP addresses is that they are very hard to remember. There's no immediately obvious logic to them, and two related computers might have completely different IP addresses. Also, for technical reasons, there are often times when the IP address of a computer has to be changed. This makes it very difficult to keep track of IP addresses.

So, a system called DNS, or Domain Name Service was created. The purpose of DNS is to translate names for computers into IP addresses. This way, computers can have a name that is easy to remember for humans, and computers can find the IP addresses that they need by consulting DNS.

This address is called an FQDN, or Fully Qualified Domain Name. A computer's FQDN can be used from anywhere on the Internet to identify the computer and translate it into an IP address. For example, WebReference.com's FQDN is www.webreference.com. Actually, this is just an alias; computers may have more than one FQDNs that point to the same IP address. You also can access the same computer with the FQDN webreference.com instead, because that too is an alias to the same computer. As a matter of fact, a computer can have many IP addresses as well, but now I'm getting too technical: the important thing to remember is that computers on the Internet have IP addresses, and FQDNs are a way to refer to them that is easier to remember; FQDNs translate into IP addresses.

Note that FQDNs are case-insensitive. www.webreference.com is equivalent to WWW.WEBREFERENCE.COM or www.WebReference.com. Most of the time, they are written in lowercase, but this doesn't mean they have to be. Also note that DNS offers a couple of ways of referring to hosts without their FQDN, but this again is something for the more technical people (in other words, if you didn't know already, chances are you don't want to know anyway).

Ending that rather large foray into the world of host naming, the hostname part of an http URL is anything that can be considered a valid hostname: an IP address, an FQDN, or one or two other cases which we won't bother with.

The third part of the URL, which is optional, is the port number. Internet hosts have a certain number of ports. You can think of them as those booths you see in a bank. Some offer one type of service (i.e. deposits), others another type of service (i.e. currency exchange) and some are just closed. It's the same with Internet hosts - one port could offer HTTP, another could offer mail routing, and so on. Ports are numbered, and most services have a pre-defined port that they usually work from. HTTP usually runs on port 80, but this is not necessary. If it is running on port 80 in the machine you named in the hostname part, then you don't need to specify a port number. If it is running on a different port, a colon (:) followed by the port number is required to point to that port.

The fourth part is also optional. It is the path to the document you are requesting. The path is a set of characters separated by slashes (/). This is roughly analogous to filenames on your hard disk. There is a root directory, directories after that that may contain directories or other other documents. Look at the following three examples:

/
/html/
/html/tutorial2/2.html
/html

The first one refers to the root directory. The second one refers to the directory /html/. The third one refers to the document 2.html in the directory /html/tutorial2/. The fourth one, by the way, refers to the document html in the root directory, and not the directory /html/; this is because it does not have a trailing slash character. Although most browsers will happily add the trailing slash when they discover that this is the case, it is still wrong to refer to directories without the trailing slash. It is perfectly possible to have both a directory and a document with the same name in a directory.

Take note that just because pathnames in URLs look a lot like pathnames in filesystems does not mean that there is a mapping between the two. Although it is sometimes the case that the directories and documents specified in URL pathnames correspond directly to directories and files in the remote computer (such as in the ftp or file URL schemes, which we'll cover later on), it is not always the case. It depends entirely on the scheme and the way this string is handled.

Also note that the path name has nothing to do with the format of the document returned. In the case of WebReference.com, all of the above examples will return an HTML document. Also, the "extension" at the end of the third example (the ".html" part) does not necessarily indicate the type of document, as it does on some operating systems like Microsoft Windows. The document http://WebReference.com/contact.php has an extension of .cgi, but is still an HTML document.

The fifth and final part of the URL is the query string, and is also optional. A query string is, essentially, input to a program that must be evaluated in a certain way. If the Web page described before the query string is such a program, then this string will make sense to it and it will return the relevant information. The query string consists of a question mark (?) followed by a piece of text that depends entirely on the program set up to handle it.

That's the full syntax of http URLs. You've probably seen a lot of them if you've been browsing the Web for any amount of time, so now you know what they mean. Now that you have a firm understanding of a URL scheme that we can use for examples, it's time to look at the concept of Relative URLs.

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Produced by Stephanos Piperoglou

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URL: http://www.webreference.com/html/tutorial2/2.html
Created: June 11, 1998
Revised: June 11, 1998