Once we have a website up and running, we may want to launch other websites. The other way to do it is to register new domain names and open new hosting accounts. So, opening new hosting accounts can be expensive,so how can we avoid this? It is possible to share the web space and bandwidth of our original account among different sites.
we can basically do so through:

* Add-On Domains
* Parked Domains, and
* Sub-Domains

An add-on domain is a new domain name that points to a subdirectory within our existing domain hosting account, where the website for the new domain will reside. Add-on domains must be registered domain names that we own, and that are configured to point to our web host’s servers.

From a web user perspective, an add-on domain functions just like any other domain. if we already have a hosting account under MAIN-DOMAIN.COM – a private hosting service, we can register and set up an add-on domain, so that when our visitors go for “http://www.add-on-domain.com” in their browser, they will be transferred to the new site. The advantage of add-on domains is that the browser’s address bar will show “http://www.add-on-domain.com” ), so the process will be totally transparent to our users. Apart from sharing web space and bandwidth with our main domain, add-on domains also get their own CGI-bin and statistics.

Many web hosts offer to set up add-on domains for free. This is only fair since we are not getting any more webspace or bandwidth. Others, however, will charge us a modest one-time fee, which is not bad, especially when the cost of registering the new domain is included. some web hosts will charge us a monthly fee for each add-on domain we set up. In some cases, that fee can be very close to the monthly cost of our web hosting account, to the point that it is better to just open a new hosting account for the new domain. If we plan to set up add-on domains in the future, we’re better off avoiding making a new domain is a kind of account.