webmasters can define how their sites need to appear in google.. there is an option of specifying if we need www or not.. its mostly a personal preference but there is also something more to it.. also pagerank was diff for site name with www and without www
I use the www with a few of my sites and dont use it with others.. the target audience for your site also come into consideration to make this decision (if you say somename.com everyone instantly recognise its a website but a lot of internet illiterate people cant recognise other tlds like say .in .us etc so a www before the name will give the internet identity in this case). anyway I will always redirect to the preferred name if some other name is typed... like a redirect to site without the www if you typed www in the address bar.. for eg. try visiting
www.wordpress.com it will redirect you to wordpress.com
nowadays the www is not required as everyone know quite a lot about the internet but still many websites use www cos of using it for such a long time and also there is no real need for not using www
a lot of it is assumption
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