URL Details

Since URL addresses are so important, let's look at their structure in a little more detail. We shall use as an example the address

http://www.whitehouse.gov/WH/Welcome.html

URL addresses have 2 basic parts: (1) a portion specifying the method of file access, and (2) a portion specifying the Internet location of the file to be accessed.

The Method of File Access

The first part of a URL, ending with a ://, tells the type of file access that is permitted from the Internet. In the preceding example http:// says that the "server" that is going to give the file to you is of the type "http", which stands for "HyperText Transfer Protocol". This part of a URL address has a function similar that of specifying the method by which a normal mail package will be transported (the U. S. Postal Service, Federal Express, UPS, ...).

The http transfer protocol is the most common one on the Web, but it is not the only one. For example, you will also commonly see URL's addressing servers using "File Transfer Protocol", or ftp:

ftp://ftp.funet.fi/pub/

and file servers using "gopher" protocol:

gopher://gopher.lib.utk.edu:70/11/Other-Internet-Resources/pictures/ut

Now, if a browser can be used to get a file (almost) anywhere on the Internet, you might think that surely it can be used to access a file on the same computer as the browser? Indeed it can, and if you do that (by choosing "Open File" from under the "File" menu) the URL will have the typical form:

file:///usr/people/guidry/webcourse/frame1.html

The URL address beginning with "file:" is the signal that the address is not off somewhere else on the Internet, but is in fact on the same computer as the browser. As a final example, you may often encounter the "mailto:" type URL address; for example:

mailto:guidry@utkux.utk.edu

In this case, "mailto:" indicates that this URL can be used to send and email message to the address specified after the colon. There are other transport protocols, but these examples illustrate some of the most important.

The Internet Location of the File

The second part of the URL address specifies the Internet address of the machine housing the file and the location of the file at that internet address. In the first example cited above, www.whitehouse.com specifies the IP address of the http server that houses the file. (See the earlier section on the Domain Naming System for the Internet.) the rest of the URL address then gives the location of the file in question on the machine specified by the preceding IP address. For example, in the URL address

http://www.whitehouse.gov/WH/Welcome.html

the file being accessed is called Welcome.html, and it resides in a directory (folder) called WH on a machine with IP address www.whitehouse.gov that is supplying the file with a server running the http protocol.

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