Most certificates are used for public websites. These websites are commonly reachable via both www.domainname.com as well as just domainname.com.
A certificate will only include a single hostname, the value that you insert into the common name of the CSR. This means that a certificate ordered for domainname.com will normally not work for www.domainname.com.
The certificate authorities have implemented a method to make sure that you do not need a multi-domain certificate or multiple certificates for a single website.
If you order a certificate from any supplier we offer, make sure you request it for www.domainname.com. After the certificate has been delivered the certificate authorities will automatically include domainname.com as an additional name in the certificate.
Using this method you will get a certificate that is valid for both hostnames that lead to your website, saving time and money on the implementation!