Sharing this as there are not many articles on it but seem to have found many questions on it.
Scenario: Currently have a Sharepoint site that runs on the Intranet on port 80. I want a separate sharepoint site private from the information shared on the Intranet that runs on a separate port and is accessible to the Internet.
Solution: Create or extend Web applications (Windows SharePoint Services)
Extend an existing Web application
You can extend an existing Web application if you need to have separate IIS Web sites that expose the same content to users. This is typically used for extranet deployments where different users access content by using different domains. This option reuses the content database from an existing Web application.
- In the SharePoint Central Administration Web site, on the Application Management page, in the SharePoint Web Application Management section, click Create or extend Web application.
- On the Create or Extend Web Application page, in the Adding a SharePoint Web Application section, click Extend an existing Web application.
- On the Extend Web Application to Another IIS Web Site page, in the Web Application section, click the Web application link and then click Change Web application.
- On the Select Web Application page, click the Web application you want to extend.
- On the Extend Web Application to Another IIS Web Site page, in the IIS Web Site section, you can select Use an existing IIS Web site to use a Web site that has already been created, or you can choose to leave Create a new IIS Web site selected. The Description, Port, and Path boxes are populated for either choice. You can choose to use the default entries or type the information you want in the boxes.
- In the Security Configuration section, configure authentication and encryption for the extended Web application.
In the Authentication Provider section, choose either Negotiate (Kerberos) or NTLM.
- In the Load Balanced URL section, type the URL for the domain name for all sites that users will access in this Web application. This URL domain will be used in all links shown on pages within the Web application. By default, the text box is populated with the current server name and port.
- In the Load Balanced URL section, under Zone, select the zone for the extended Web application from the drop-down menu. You can choose Intranet, Internet, Custom, or Extranet.
- Click OK to extend the Web application, or click Cancel to cancel the process and return to the Application Management page.