If you have sub-domains linked to the main domain, you can also use the WildCard SSL certificate. A single WildCard SSL certificate reduces the management costs to a great extent. It creates a secured private connection between the web browser and web server that covers the web forms, gateways, mail, FTP servers and VPNs. This is one of the most secured and efficient ways of securing the sub-domains linked to the website. Here is a complete explanation on the WildCard SSL –
WildCard SSL Certificate.