Server Name Indication (SNI) is an extension to the TLS computer networking protocol by which a client indicates which hostname it is attempting to connect to at the start of the handshaking process. This allows a server to present multiple certificates on the same IP address and TCP port number and hence allows multiple secure (HTTPS) websites (or any other Service over TLS) to be served off the same IP address without requiring all those sites to use the same certificate. It is the conceptual equivalent to HTTP/1.1 name-based virtual hosting, but for HTTPS.
To make use of SNI practical, the vast majority of users must use web browsers
that implement it. Users whose browsers do not implement SNI are
presented with a default certificate and hence are likely to receive
certificate warnings, unless the server is equipped with a wildcard certificate that matches the name of the website.