NOTE: The Server Name Indication (SNI) protocol extension is supported starting from IIS version 8.0. Therefore, the instructions below are not applicable for older versions of IIS. Legacy servers can only use one SSL certificate per IP address. If you have separate certificates for each of your websites, it will be necessary to assign a dedicated IP address for each site. An alternative approach is to use a multi-domain (also known as SAN) certificate that covers the required hostnames.
How to Install Multiple Certificates on IIS
Step 1
During certificate installation steps make sure that you tick Require Server Name Indication as shown on the screenshot:
Step 2
Server Name Indication (SNI) is a specific SSL/TLS extension that allows using multiple SSL certificates on a single IP address.
Generally, the best practice for IIS servers is to have *one* SSL certificate that can be considered as the default certificate on the server. All websites that use that default SSL should have the Require Server Name Indication box unchecked.
All additional certificates must be used along with SNI enabled; otherwise, this may cause errors in IIS and disrupt the work of your websites.