Knowledge Base

Subject Alternative Name (SAN)

Updated on August 14, 2026

Overview

The Subject Alternative Name (SAN) field is an extension in an SSL certificate that allows additional names to be protected by a single certificate. These names can include host names, websites, IP addresses, common names, and other identifiers.

SANs are commonly used with Multi-Domain SSL certificates because they allow multiple domains or services to be secured under one certificate instead of requiring a separate certificate for each name.

Background

The Subject Alternative Name extension was part of the X.509 certificate standard before 1999. SAN usage became more common after Microsoft Exchange Server 2007, where SANs helped simplify server configurations that required multiple secure names.

Today, SANs are widely used in environments that need to secure multiple sites, domains, or subdomains with one SSL certificate.

What Can a SAN Secure?

A Subject Alternative Name can be used to secure different types of names in a single SSL certificate.

Examples include:

Fully qualified domain names, such as www.example.com

Additional domains, such as www.example.net

Subdomains

Host names

IP addresses

Common names

How SAN Certificates Differ from Wildcard Certificates

A Wildcard Certificate can secure first-level subdomains under one base domain, such as *.example.com. However, a Wildcard Certificate cannot secure unrelated domains such as www.example.com and www.example.net in the same certificate.

A Multi-Domain certificate that uses SANs can secure names across different domains in one certificate.

Certificate Type	What It Can Secure	Example
Wildcard Certificate	Multiple first-level subdomains under the same base domain	*.example.com
Multi-Domain SAN Certificate	Multiple domains, subdomains, or host names in a single certificate (includes up to three domains by default; additional SANs can be purchased as needed).	www.example.com, www.example.net, mail.example.org

Common Uses for SANs

Secure Multiple Domains with One Certificate

SANs allow one certificate to protect names on different base domains. This is useful when an organization manages multiple websites or services that need SSL protection.

Licensing and SAN Coverage

Multi-Domain SSL certificates typically include coverage for up to three domains by default. If you need to secure additional domains beyond the included limit, extra SANs can be purchased either during the initial order process before checkout or added later during certificate reissuance with assistance from the Sales team. Each additional domain added to the certificate consumes one SAN entry and may be subject to an additional cost. Additional SANs can be purchased to secure up to 250 domains in total under a single certificate.

Payment Page:

To secure additional domains, extra SANs can be added using the option below on the payment page before checkout.

Host Multiple SSL Sites

Multi-Domain certificates with SANs can help support multiple SSL-enabled sites on a single server. The source article states that Microsoft IIS and Apache can use Multi-Domain SAN certificates for HTTPS virtual hosting.

Simplify SSL Configuration

Using a Multi-Domain SAN certificate can reduce the need to configure multiple IP addresses, bind separate certificates to each IP address, and manage multiple individual certificates.

How to View SAN Entries in a Certificate

To view SAN entries, open the certificate details in a browser and look for the Subject Alternative Name extension. The SAN section lists the names protected by the certificate.

Figure: Example of certificate details showing Subject Alternative Name entries.

Example

If both www.example.com and example.com are listed as SAN entries in the certificate, a browser can validate the certificate for either name. The source article gives a similar example using www.COMODO.com and COMODO.com.

Key Points

A Subject Alternative Name allows one SSL certificate to secure multiple names.

SANs can include host names, websites, IP addresses, and common names.

SANs are commonly used with Multi-Domain SSL certificates.

SANs can simplify SSL configuration for environments that use multiple secure sites or services.

Wildcard certificates and SAN certificates solve different use cases. A wildcard certificate secures subdomains under one base domain, while a SAN certificate can secure names across different domains.