Hi

I think, we have to deal with two separate issues here

1. The protocol

As nihil pointed out, the text before the :// refers to some protocol and/or
tells the browser what to do:
Like (sorry for paraphrasing):
http: check on the following address port 80
https: check on the following address port 443
ftp: check on the following address port 21
file: check on the localmachine for the file ...

2. the address
(apart from file://)
The text after ://...

The whole thing that follows, like www.google.com or google.com, points to an address.
The IP address is different (test it using ping), but both sites will return the google
search engine mask on port 80.

The subdomain-IP (subdomain.domain.com) addresses are part of the DNS entry on the DNS server managing
your domain (simply spoken).

You can also forward all subdomain-IP's to one and the same IP.
There, using http1.1, you can configure a web-server (eg apache)
send different "homepages" according to that address ("virtual hosting").
like
joe.myplace.com
will point to another directory than
john.myplace.com.


And, as nihil already said (sorry for repeating again )
nowadays, the browsers interpret what you have typed to the most
probable choice. like
google -> http://www.google.com

www is a standard for a website.
other standards are/were/would be

ftp.google.com -> would resolve to the ftp server
mail.google.com -> would resolve to the mail server
...

Cheers