When trying to navigate to our site in a browser, why does my custom domain not work if I enter the address with a leading 'www.' first?

Environment:

SSL certificates for custom domains
Customer Portal (CP)

Issue:

While in a browser trying to access our end-user pages, entering a leading 'www.' before our custom domain does not work. The link works fine normally, but does not with the leading 'www.' added to the address. 

Solution:

This can occur because the leading www. is not assumed in the hostname or on the Internet, and must be explicitly defined on the certificate that covers your site. This can be done by adding a Subject Alternative Name to the certificate that defines a www. version of the desired hostname as well. 

In other words, if you need examplesite.com as a common name on a certificate, but also need coverage for www.examplesite.com, then you will need to define www.examplesite.com as a Subject Alternative Name.

This cannot be added afterward once the certificate is already active, so you are encouraged to carefully review all addresses that you need coverage for prior to purchasing your certificate. 

Additional Information:

Answer 2031: Using SSL and your own domain name in the URL for a site

Answer 7988: Managing SSL Certificates using Oracle B2C Service Configuration Assistant