SSL Certificate Subject Alternative Names (SANs)
Subject Alternative Names or SANs allow you to secure multiple domains from one SSL/TLS certificate. SANs are additional domain names added to an SSL certificate.
SANs are the ideal solution to secure Microsoft Exchange and Office Communications Servers with Unified Communications Certificate (UCC), also known as a SAN or Multi-domain certificate.
For example, an SSL certificate’s main domain (Common Name) could be www.domain.com with three included SANs of secure.domain.net, shop.example.info, and mail.domain.com. Then, when any of those four domains are requested over HTTPS in a browser, the end-user won't get any browser certificate-mismatch errors.
SANs are also very useful as a "fix" for the www versus no-www browser problem. If your certificate is issued for www.domain.com it will only work properly in a web browser for exactly www.domain.com and not just the base, domain.com (without the www). Adding a SAN value of domain.com to the certificate resolves this problem. The converse of this works too; that is, you can add a SAN value of www.domain.com for certs issued just domain.com (without the www).