Uniform Resource Identifiers (URIs)

Every resource available on the web, including HTML files, graphics, scripts, movies, and so forth, needs a unique address. The addressing system that identifies the path to each resource on the Internet is called Uniform Resource Identifiers (URIs). Like other networks, such as your phone system, the URI system works on the idea of sets and subsets. In the telephone world, the set consists of:

You can only have one country code 01 in the entire network. But, you can have one area code 978 in country code 01, and one area code 978 in country code 02, and so forth. Every subset of numbers, then, is entirely repeatable in the next higher set of numbers, giving literally millions and millions of possible combinations. Yet each telephone in that network, whether in Boston Massachusetts or Hamburg Germany, is uniquely addressible.

In the Internet world, the "telephone numbers" for the network are IP addresses, a numeric addressing scheme that has four subsets of numbers ranging from 0-255, such as:

198.144.128.3

To make the Internet more

Uniform Resource Identifiers (URIs) on the Internet work along the same idea, only there are two main components:

• IP address/Domain Name (the numeric or alphabetic address that routes to a specific computer on the Internet)
• File name (which might include the directory/subdirectory structure as well as the file name).

To make the Internet easier to work with, the Internet uses the Domain Name System to allow for English-like addresses rather than numeric addresses. So, for example:

URI: http://www.empire.net/index.html
(with Domain Name)

is the same as

URI: http://198.144.128.3/index.html
(with IP address)

NOTE: The example above points to the ISP that hosts this course, with the logic that if their systems are down you won't be reading this example anyway!

And Internet router translates EMPIRE.NET to IP address 198.144.128.3 and routes communications to the computer that is the web server for EMPIRE.NET. Once there, the remainder of the URI is the filename INDEX.HTML.

Just as with phone numbers, the numbering scheme for IP addresses involves sets and subsets of numbers. Each subset can be numbers 0 through 255. That may not sound like many options, but with the combination of 4 subsets that equals approximately 4.3 billion combinations!

So, if every computer on the internet has a unique address, what is your computer's IP address? Well, we've checked it out...you should see it displayed at the lower-left side of your browser window, in the "status bar"...

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