Answered: When using the ssl cert for a single domain will all sub-domains be protected using a single ssl cert?

Hello,

This depends on the type of certificate you purchase.  If you have 3 or less subdomains then a multidomain certificate will work for you without incurring additional fees.  

The following certificates will work with three different sub-domains:

https://www.ssl.com/certificates/premiumssl
https://www.ssl.com/certificates/ucc

If you need more than 3 subdomains then this product would be much better:

https://www.ssl.com/certificates/wildcard