Subdomains do not work without the leading www

What is the difference between the following URLs?

• http://www.forums.example.com and
• http://forums.ex开发者_Go百科ample.com ?

Actually I've created a new subdomain in my website and it's working fine when I try to access without www (i.e http://forums.example.com) But when I try to give URL like http://www.forums.example.com it is showing "Page not found".

Why is this and how can I fix the issue?

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If I am not mistaken, www. is also considerd a Subdomain, hence when you setup hosting on a provider (i.e Slicehost) they tell you to put www in the subdomain list (again if I am not mistaken). If you really want www.fourms.duckyetc you can set up a script to do some matching, i.e

if url contains www.fourms.ducketc then go to here else go somewhereelse

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You have to create a cname for www.forums.duckyvideos.com and also configure your webserver to respond to that cname. There is nothing automatic about www prefixes on the web. It's just a convention that websites are accessible with or without the www. Everyone hosting a website has to explicitly set up a second cname for the www prefixed version of every site they put up.