SNI is an acronym that stands for "Server Name Indication." It is a protocol extension used in Transport Layer Security (TLS) and Secure Sockets Layer (SSL) protocols. SNI allows multiple websites to be hosted on a single IP address, by letting the client indicate the name of the desired website during the handshake process.
Traditionally, when using SSL or TLS, a single IP address could only support one website. This limitation posed challenges for hosting multiple websites on a single server, often requiring the use of additional IP addresses. However, with the introduction of SNI, this constraint was eliminated.
When a client initiates an SSL/TLS handshake with a server, it includes the hostname of the desired website in the SNI extension field of the ClientHello message. This informs the server which certificate to use for the requested website and allows it to present the correct website to the client.
SNI has become particularly crucial with the growth of virtual hosting, where multiple websites are hosted on the same physical server. By relying on SNI, server administrators can offer secure connections to multiple sites without requiring extra IP addresses.
While SNI support is now widely adopted by modern web browsers and servers, some older browser versions do not support SNI. Therefore, server configurations still need to consider fallback options for providing compatibility with those older clients.
