What is a Subdomain??

A subdomain is a domain that points to a specific directory in the main domain's document root. For example: If your main domain is cpanel.net and you had a section about Support or your site, you could create the subdomain http://support.cpanel.net. Now, whenever someone goes to http://support.cpanel.net/ they will be taken to the directory that the subdomain points to. In this case, it would be /support/. This means that http://cpanel.net/support and http://support.cpanel.net/ would take you to the same place.
Your current subdomains will be displayed below Current Subdomains. These correspond with the directory that is the same name as the subdomain prefix. The prefix is the part before the domain name. For example: The prefix for http://support.cpanel.net is support so the contents of http://support.cpanel.net will be in /public_html/support/
A great advantage to a subdomain is that it does not have to be registered since it is technically just a directory of the main domain. You can create a subdomain, for example: http://sub.domain.com and it will work instantly and does not have to be put on file with any domain registrar to work.
Creating Subdomains
To add a new subdomain just type the prefix in the blank field beneath Create A Subdomain and then select the domain it should be added as a prefix for from the drop list next to the blank field. The domains in this list will include your main domain name and any addon or parked domains you may have. After selecting the domain it will be added as a prefix for, click on Add.
While a subdomain is contained in a directory, just creating a directory will not create a subdomain. You need a DNS record for the subdomain which is created through the above steps. If you make a directory without adding it as a subdomain, it will only work with http://domain.com/directoryname not http://directoryname.domain.com
Removing Subdomains If you no longer wish for a directory to be accessible from directoryname.domain.com, you can remove the redirection created by adding that subdomain. This will not delete the contents of the directory. If you wish to remove the directory all together, remove the redirection and then delete the directory through the File Manager or through FTP.
FTP Access
f you wish to grant virtual ftp access to this subdomain, just create an FTP user with the same username as the subdomain base name that is listed in the Subdomain column below. For example, if your subdomain is bill.domain.com, create the ftp user bill to access the subdomain.
Subdomain Resolution
Subdomains rely on the DNS system in order to function, which means that your DNS information must propagate throughout the different DNS servers on the Internet before your subdomain becomes fully active. This process on the average takes a few hours.