Assign a second IP address to the server network card, create a folder on the
server;e.g., c:\extranet, and create a new website in IIS and make the home
directory c:\extranet. Make the FQDN DNS point to the IP Address for the
extratnet web site. Set the site security to require SSL and set the port for
the site to 443 only - no port 80.
In SharePoint Central Administration -> Configure alternate portal site URLs for
intranet, extranet, and custom access. The default web site should vbe listed.
Edit the entry using the dropdown. Leave the default setting and put your
https://yourFQDN as the extranet.