!!! Overview
[{$pagename}] ([SNI]) is an [TLS extension] computer networking protocol by which a client indicates which hostname it is attempting to connect to at the start of the handshaking process. 

[{$pagename}] allows a server to present multiple [certificates] on the same [IP Address] and [TCP] port number and hence allows multiple secure ([HTTPS]) [websites] (or any other Service over [TLS]) to be served off the same IP address without requiring all those sites to use the same [certificate]. [{$pagename}] is the conceptual equivalent to HTTP/1.1 name-based virtual hosting, but for [HTTPS].

To make use of [{$pagename}] practical, the vast majority of users must use web [browsers] that implement it. Users whose browsers do not implement [{$pagename}] 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.

[Encrypted Server Name Indication] for TLS 1.3 adds [Encryption] to [{$pagename}]

!! More Information
There might be more information for this subject on one of the following:
[{ReferringPagesPlugin before='*' after='\n' }]
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* [#1] - [Server Name Indication|Wikipedia:Server_Name_Indication|target='_blank'] - based on 2015-05-13