The Subject Alternative Name field explained
Subject Alternative Names let you protect multiple host names with a single SSL certificate.
Subject Alternative Names allow you to specify a list of host names to be protected by a single SSL certificate.
The Subject Alternative Name extension has been a part of the X509 certificate standard since before 1999, but only recently achieved widespread use with the launch of Microsoft Exchange Server 2007—which makes good use of Subject Alternative Names to simplify server configuration.
What can you do with Subject Alternative Names?
Secure host names on different base domains in one SSL Certificate. A wildcard certificate can protect all first-level subdomains on an entire domain, such as *.example.com. But a wildcard cannot protect both www.example.com and www.example.net.
Virtual Host Multiple SSL sites on a single IP address. Hosting multiple SSL-enabled sites on a single server typically requires a unique IP address per site, but a certificate with Subject Alternative Names can solve this problem. Microsoft IIS 6 and Apache are both able to Virtual Host HTTPS sites using Unified Communications SSL, also known as SAN certificates.
Greatly simplify your Exchange Server 2007 SSL configuration. Using a SAN certificate saves you the hassle and time involved in configuring multiple IP addresses on your Exchange 2007 server, binding each IP address to a different certificate, and running a lot of low level PowerShell commands just to piece it all together.
Where can you see Subject Alternative Names in action?
To see an example of Subject Alternative Names, visit this page using https and then click the padlock in your browser to examine our SSL certificate. In the certificate details you will find a Subject Alternative Name extension that lists both www.digicert.com and digicert.com.
Because the name digicert.com is listed in our certificate, your browser will not complain if you visit our site at https://digicert.com without the 'www' in the name.