Navigation with Browsers

URL Details

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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/kids/index2.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 to 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 may also see URLs addressing servers using "File Transfer Protocol", or ftp:

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

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" 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. Actually, the above would be the form of the URL for a file on a Macintosh or a Unix machine. For a file on a PC, you would see a URL of the form

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

since a PC designates a disk drive, or other file storage device, with a letter and colon, rather than with a name. As a final example, you may often encounter the "mailto:" type URL address; for example:

mailto:[email protected]

In this case, "mailto:" indicates that this URL can be used to send an 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.gov 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/kids/index2.html

the file being accessed is called index2.html, and it resides in a directory (folder) called kids on a machine with IP address www.whitehouse.gov that is supplying the file with a server running the http protocol.