Subdomain is an extension of your main domain. For example if your domain is example.com, you could create subdomains singapore.example.com and tokyo.example.com. These subdomains could be accessed just like your main domain.

Our email system is also subdomain aware. You can create email for subdomains hosted on your account. Based on the example above, you could create john@singapore.example.com and john@tokyo.example.com, both are different users.

As you can see, subdomain is a nice way of organizing your your Internet site in a more professional and structured way.