The primary reason most people use shared servers is its low-cost.  Shared servers are usually the best choice for entry-level web-sites or for small businesses whose web-sites do not have high traffic levels.
Most shared-server web hosting companies that charge under $10.00/month have limited or no technical support available due to their low profit margin.  Eight Dollar Web Hosting is the exception to this.

A dedicated server is a single computer, fully devoted to the needs of one customer. It allows for faster access to information and provides greater flexibility in software, site traffic management, and scalability.  It is a more significant expense than sharing a server and distributing the cost and providing a more economical solution.

When a site requires more than 500 MB of disk space or 20 GB of data transfer in a month, a dedicated solution can be more cost effective than buying disk space by the Megabyte, or paying for additional bandwidth as an add-on to shared hosting.  Another advantage of using a dedicated server is the ability to install and use special, custom software applications on your server that the shared server doesn't offer.

Dedicated servers can command a monthly fee of $299.00.
 

100MB Business Plan
$96.00/year
includes
Up to 100 Megabytes of disk space
5GB of monthly bandwidth
Ad-free website
Web-based or POP email
Web Statistics

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300MB Corporate Plan
$144.00/year
includes
Up to 300 Megabytes of disk space
10GB of monthly bandwidth
Ad-free website
Web-based or POP email
Web Statistics

MORE