The Domain Tables, available for lookup purposes to all the computers on the Internet, are constantly being updated and propagated so that any address can be found by any computer. A server address need not have just three parts. The web page at the end of the URL is the actual HTML document that gets loaded in your web browser. They are separated by a dot. Now most web pages are created on PCs running Windows 95 and the three letter extension is the standard.
All web browsers understand both conventions.
I think I understand this long winded explanation. That seems to be much simpler than the one you explained above. The above explanation was for the sake of completeness. Many large information providers, like cnn, want to make it as simple as possible for you to find their sites, so that create a short, simple address. It only seems to have the protocol and the name of a server.