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SubDomain - The Third Level Domain
Published: Friday, March 12, 2004
What is a subdomain?
A subdomain is the part of the website address before the domain name. For example, the address for WebsiteGear forum is http://forum.websitegear.com. Here, forum is a subdomain of the domain name websitegear.com. Subdomains are also known as the third level domains or canonical names. A subdomain, unlike a domain name, is not registered anywhere because it is associated with a domain name only. It can be created by the web host on the DNS server. The most commonly used subdomain is www, as in http://www.websitegear.com. However, there is no need to add www in front of the domain name. Similarly, mail server addresses often have mail as the subdomain, as in mail.websitegear.com.
Why are subdomains used?
Subdomains are commonly used to categorize portions of the website. For example, the services offered by WebsiteGear are categorized by their subdomains, such as poll.websitegear.com or rating.websitegear.com. The benefit is that the subdomain can be easily moved to another server if the category gets very popular.
Subdomains are also used by free webhosting providers to resell web space under their own domain name (e.g. http://membername.hostname.com). Each member will have their subdomain, however, they all will still share the domain name of the hosting provider.
The third reason for using a subdomain name is to load balance the web servers for a high traffic website. Multiple web servers are assigned different subdomains like www.sitename.com, www1.sitename.com, www2.sitename.com etc, though each of them contain the same application code. When the request comes from the browser, the load balancing software redirects it to one of these servers. DNS load balancing is a simple method of load balancing using subdomains pointing to different IP addresses.
Types of subdomain setup
Subdomain can be setup in different ways, although the subdomain information is stored similarly in the DNS server.- Separate Site: This type of subdomain setup will the have a separate hosting account or hosting location with respect to the domain hosting account. The hosting provider usually assigns new resources (disk space, bandwidth etc.) for this subdomain and charges fees as if it is another hosting account. There is often no way to share common files (such as include files by relative path) between the domain account and subdomain account.
- Sub-Directory Pointing: In this case, the subdomain is pointed to a sub-directory of the web root folder. For example, subdomain.domain.ext will point to /account/www/subdomain directory. Essentially, the resources of the domain in this case are shared by the subdomain as well. Usually the hosting provider does not charge additional fees (other than subdomain setup fees). Both the domain and subdomain can also reuse the same common files (such as SSI includes) from the same shared directory location.
- Separate Server: This type of configuration is similar to the separate site configuration but hosted on a separate server with a different IP address. In this case the subdomain can reside at a totally different geographical location. The advantage of having a separate server for a subdomain is in load distribution among servers (by separating application functionality) or providing horizontal scaling in case of load balancing between servers (by identical replication of application code on each server).
More On Sub-Domains
More detailed discussions on subdomains and cookie handling within subdomains is provided in the next section. Click here.
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