IIS problem: company site works with www prefix but doesn't work without them

[RMSistight]

When I launch a browser with "www.companyname.com", the website registers.

But, when I launch a broswer with "companyname.com", I get this message:

The page must be viewed over a secure channel
The page you are trying to access is secured with Secure Sockets Layer (SSL).
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Please try the following:

Type https:// at the beginning of the address you are attempting to reach and press ENTER.
HTTP Error 403.4 - Forbidden: SSL is required to view this resource.
Internet Information Services (IIS)

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Technical Information (for support personnel)

Go to Microsoft Product Support Services and perform a title search for the words HTTP and 403.
Open IIS Help, which is accessible in IIS Manager (inetmgr), and search for topics titled About Security, Secure Sockets Layer (SSL), and About Custom Error Messages.
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How do I go about to fix this?

 

[bsobel]

First, the obvious, are you sure both names resolve to the same IP?

Second, are you using host headers? Did you set the host header on the site for both forms of the name?