A Wildcard SSL certificate secures your website URL, and an unlimited number of its subdomains. A single Wildcard certificate can secure domain.com, www.domain.com, mail.domain.com, and any other subdomain.domain.com.
Wildcard certificates secure all of the subdomains at the level you specify when you submit your request. Just add an asterisk (*) in the subdomain area of the common name where you want to specify the wildcard. For example:
If you configure *.domain.com, you can secure
If you configure *.www.domain.com, you can secure
Wildcard certificates secure websites the same as a regular SSL certificate, and requests are processed using the same validation methods. However, some Web servers might require a unique IP address for each subdomain on the Wildcard certificate.
NOTE: A Wildcard certificate secures only the level of subdomain you specify. So, if a certificate is configured for *.www.domain.com, it will not secure www.domain.com.