What is SNI?
When using openssl, and thus TLS, the SNI name can be a DNS name or an IP address (OpenSSL.org). TLS needed the SNI extension capability for a client server to send the hostname in the first (a "client hello" message) of three parts of the TLS handhsake process (OpenSSL.org). The second part is a "server hello" message from the server (Microsoft). The final part of the handshake is the "server hello done" message from the server again (Microsoft). To use TLS to learn more about how it works, see this link.