[CiaranMcNulty]

Historically, you can't serve multiple sites from one IP address (i.e. named virtual hosts) and use HTTPS. The reason for this is that the hostname of the site is included in the HTTP request from the client:

  HTTP/1.1 GET /
  Host: mysite.com

By the time the server has decoded and read this header, you have presumably already started the secure connection, so the server has to have already selected which certificate to use for the session.

Workarounds are to have multiple IP addresses on your box with one cert per IP, or run the server on multiple ports with one cert per port. In both cases this enables the server to know which certificate to use from the underlying connection properties, and not wait for the encoded traffic to start arriving.

SNI (Server Name Identification) is an extension to TLS (Transport Layer Security) that essentially adds the hostname into the SSL negotiation, so this cert can be selected by the server in advance. It made it into OpenSSL implementations in the mid-2000s and is reasonably widely adopted.

Legacy libraries, Internet Explorer <=7, and Windows <=XP won't support it so it's not quite ready for mainstream use. Give it 5 years or so...