Subject Alternate Name (SAN) / Unified Communication (UC)
Subject Alternative Names protect multiple host names with a single SSL certificate. It allows specifying 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 Name to simplify server configuration.

Secure host names on different base domains in one certificate. 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 certificate, also known as SAN certificates.

SAN Certificates can secure: Multiple fully qualified domain names (FQDN) with a single certificate. SAN Certificates are often needed to secure Exchange 2007 Server or Office Communications Server 2007.

Instances where there is a need to secure multiple domains that resolve to a single IP address (such as in a shared hosting environment).

Using a SAN certificate saves the hassle and time involved in configuring multiple IP addresses on 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.

When browsers connect to server using https, they check to make sure SSL certificate matches the host name in the address bar.
There are three ways for browsers to find a match:
  • The host name (in the address bar) exactly matches the Common Name in the certificate's Subject.
  • The host name matches a wildcard common name. For example, matches the common name *
  • The host name is listed in the Subject Alternative Name field.
Comparing the Server Name it connects to with the Common Name in the Server certificate is a common way SSL client (Browser) match the host name typed in the address bar. It's a safe to assume that all SSL clients support exact common name matching.

If an SSL certificate has a Subject Alternative Name (SAN) field, then SSL clients are supposed to ignore the common name value and seek a match in the SAN list.
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