What is SNI (Server Name Indication)?

When you access a secure website, your browser and the server work together to keep your data safe. But how do servers know which website’s SSL certificate to use, especially when many sites share the same IP address? That’s where Server Name Indication (SNI) comes in.

This guide explains the meaning of SNI, why it matters for HTTPS websites, and how it helps with shared hosting. You’ll also learn how SNI works, its benefits, common compatibility questions, and what encrypted SNI means for privacy. Let’s start by answering the “What Is SNI?” question to make sense of this hidden but powerful part of secure web browsing, then cover the tls sni, ssl sni, sni meaning.

KEY TAKEAWAYS

• SNI is part of the TLS handshake used in HTTPS connections.
• It allows a browser to tell the server which website to visit before the secure connection begins.
• SNI helps servers choose the correct SSL certificate, even when hosting many websites on the same IP address.
• Almost all modern browsers and devices support SNI by default.
• Encrypted SNI (ESNI) and Encrypted Client Hello (ECH) are new versions that improve privacy by hiding the website name during the handshake.
• SNI makes secure web browsing easier, cheaper, and more flexible.

What is SNI & Why Does It Matter?

Before we explain Server Name Indication (SNI) in detail, let’s take a quick overview. It was added as an extension to the SSL/TLS protocol in 2003, meaning it was not originally part of that protocol. Nearly all modern browsers, operating systems, and web servers support SNI. However, a few old browsers and systems still in use may not be compatible.

SNI plays an important role in how secure websites work today. When you visit a site using HTTPS, your browser and the server must agree on secure communication. This process is called an SSL/TLS handshake, which happens before any data is shared. For the handshake to work properly, the server must show the correct SSL certificate that matches the website you’re visiting.

Before SNI was introduced, servers had a major limitation. They could only show one SSL certificate per IP address. This meant that if multiple websites were hosted on the same server, only one could have a secure HTTPS connection, unless each had a separate IP address. This became a significant problem for shared hosting, where multiple sites share the same server to save money and resources.

The biggest issue appeared during the handshake process. The browser would try to connect securely, but the server didn’t yet know which website the user wanted to access. As a result, it might show the wrong certificate, which could lead to connection errors or security warnings.

SNI solves this problem by allowing the browser to tell the server the website name (domain name) it’s trying to visit at the very start of the handshake. With this data the server can present the correct SSL certificate for the right site.

To make this easier to understand, think of an apartment building. If you send a package but only write the building’s address, the delivery person won’t know which door to knock on. But if you include the apartment number, the package goes to the correct address. SNI tells the server which website (apartment) the browser is requesting.

Also, SNI helps prevent a common name mismatch error, which happens when a user’s device connects to the correct IP address, but the SSL certificate shown doesn’t match the website’s domain name.

This mismatch often triggers a browser error, such as Your connection is not private, causing users to lose trust or leave the site. By sending the correct hostname during the TLS handshake, SNI ensures the server presents the right certificate, avoiding these errors.

How Does TLS SNI Extension Work?

SNI is used during the TLS handshake, where your browser and a server agree on how to connect safely using HTTPS. For this to work, the server must present the correct SSL certificate that matches the website you wish to visit.

Before SNI, when multiple sites shared the same IP address, servers couldn’t determine which website a user requested. This often caused the wrong SSL certificate to load, leading to browser errors or security warnings.

SNI solves this by allowing your browser to send the hostname (such as example.com) in the first message of the handshake, called the Client Hello. The server reads this hostname, picks the matching SSL certificate, and then continues the handshake to create a secure connection.

Here’s how it works step-by-step:

1. You visit a website that uses HTTPS.
2. Your browser starts the TLS handshake and includes the website’s hostname in the Client Hello message.
3. The server checks the hostname and finds the correct SSL certificate.
4. It sends back the right certificate, matching the site you’re visiting.
5. The secure connection is completed, and your browser loads the page with HTTPS.

Thanks to the TLS/SSL SNI extension, one server can securely host many HTTPS websites with the same IP address. It helps save costs, simplifies hosting, and makes secure browsing smoother for users everywhere.

Server Name vs Hostname vs Virtual Host

A hostname is the website’s domain name that a user wants to visit. This hostname may differ from the actual name of the server hosting the site. In many cases, a single web server hosts multiple domains, each referred to as a virtual hostname.

A server name refers to the name of the machine running a web server. This name remains hidden for most users, unless the server hosts just one domain, in which case the server name and domain name may be the same.

What is SNI & Dedicated IP SSL?

When setting up HTTPS for your website, you have 2 main options: SNI SSL or Dedicated IP SSL.

With a dedicated IP SSL, each website needs an IP address to install an SSL certificate. This was the only method available before SNI. It works well, but it can be expensive and uses up limited IP addresses.

SNI SSL, on the other hand, allows many websites to share one IP address and still have their own SSL certificates. It’s a more modern and budget-friendly choice, especially for WordPress Shared Hosting plans.

Here’s a simple comparison:

Feature	SNI SSL	Dedicated IP SSL
Cost	More affordable	More expensive (extra IP cost)
Setup	Easy and automatic on most hosts	Many need manual setup
Compatibility	Works on modern browsers	Works on both old and new

When your visitors use updated browsers and devices, choose SNI SSL. However, if your site supports old systems, a Dedicated IP SSL may still be necessary.

Encrypted SNI & Encrypted Client Hello

SNI helps servers choose the right certificate, but it has one privacy issue: it sends the hostname in plain text so third parties, such as network providers, see which website you’re visiting, even if the content is encrypted.

To fix this, Encrypted SNI (ESNI) was introduced. ESNI hides the hostname during the TLS handshake so others can’t see it. This protects your privacy from anyone tracking or filtering websites based on names.

An even better version called Encrypted Client Hello (ECH) is being rolled out. ECH doesn’t just encrypt the hostname; it also secures the entire first message of the TLS handshake. This gives users stronger privacy and security when visiting websites.

While ECH is still new, some browsers and services are starting to support it. As adoption grows, online privacy will continue to improve for everyone.

How to Check if a Website Uses SNI

After learning “what is SNI?” in detail, if you’re curious whether a website uses SNI, there are a few simple ways to find out. One easy method is to use the SSL Labs SSL Test tool. Enter the domain name and click Submit to run the test. Once the results appear, check for SNI support in the report. If the website uses SNI, this will be mentioned in the results.

You can also check if a single IP address serves multiple SSL certificates for different websites. If this is the case, it’s a strong sign that the server uses SNI. Checking for SNI is useful for web developers, security testers, and anyone interested in how HTTPS works behind the scenes.

FAQS

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– SSL Certificate Chains How They Work & Why They Matter

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