Why www Prefix for any website?

In the beginning every computer on the Internet had a unique address.
To send a message to one of them you worked from right to left so you
sent a message to .com which knew the address of mysite.com which knew
the address of fredscomputer.mysite.com. A convention arose so that
you could expect a service to be provided by a computer that had a
suitable name. ftp.mysite.com handled all the FTP file transfers and
pop3.mysite.com handled all the incoming mail. Similarly, the computer
named www located at mysite.com is expected to handle all the http
requests.

The concept is simple but since there are a few computers on the way
they are likely to do some translations for you. If you enter
mysite.com into your browser it may translate it to www.mysite.com
before it accesses the Internet. A computer at mysite.com, or along
the way, could send all http requests to the machine called
fredscomputer.mysite.com but that computer can be programmed to
display the location as www.mysite.com. It may forward some requests
to another computer called www2.mysite.com. To add to the confusion
there can be a few hundred computers cooperating to look like one
computer called, for example, www.google.com.

Courtesy :
http://www.velocityreviews.com/forums/t163812-why-www.html

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