Subject Alternative Name Certificates are public certificates with a list of alternative domains, sub domains and/or IP addresses that can also use the certificate. For example CrucialExams.com, www.CrucialExams.com, api.CrucialExams.com and the IP 22.214.171.124 all in a single cert. Wildcards are a close alternative that supports any sub domain (e.g. *.google.com) but a wildcard could not also be used for gmail.com. To use a single certificate for a sub-domain and entirely different domain a SAN must be used.
Subject Alternative Name (SAN) is an extension to X509 that allows various values to be associated with a security certificate using a subjectAltName field These values are called Subject Alternative Names (SANs) Names include: Email addresses IP addresses URIs DNS names: this is usually also provided as the Common Name RDN within the Subject field of the main certificate Directory names: alternative Distinguished Names to that given in the Subject Other names, given as a General Name or Universal Principal Name: a registered object identifier followed by a value