Subject Alternative Names or SANs allow you to secure multiple domains from one SAN certificate hosted on the same server. SANs are additional domain names added to an SSL certificate.
SANs are sometimes useful to secure Exchange Servers or Office Communications Server and in instances where you need to secure multiple domains that resolve to a single IP address (such as in a shared hosting environment).
For example, an SSL certificate’s main domain could be www.domain.com with three included SANs of secure.domain.com, shop.domain.com, and mail.domain.com. Then, when any of those four domains are requested over HTTPS in a browser, the end-user shouldn’t get any browser certificate-mismatch errors.
SANs are also very useful as a to fix the www versus no-www problem. If you are issued an SSL certificate for, say, www.domain.com it will only work properly for exactly www.domain.com and not just domain.com (without the www). Adding a SAN value of ‘domain.com’ to the cert fixes this problem. This works conversely too, that is you can add a SAN value of www.domain.com for certs issued just domain.com (without the www).
QuickSSL Premium allows for an additional three SANs. Each SAN in a QuickSSL Premium cert must have the same registered domain. IMPORTANT! SAN names in a QuickSSL Premium cert CANNOT be added, amended, or deleted once the certificate is issued.
The True BusinessID Multidomain (MD) SSL product allows for up to 24 SANs. SANs in this type cert can be a mix of internal and external domains, server names and private IPs. SANs in the MD product can be changed at any time during the life-cycle of the MD certificate.