because it does not take up very much computational power to serve a small or medium sized web site, one computer can handle many such sites at a time. Originally though, it was only possible to run one web site per computer; a very expensive proposition. However, software was developed that allowed one computer to serve multiple web sites. Thus, each site runs under its own virtual server, which is essentially just a separate program running on the computer. In this way, many more people can be served by a much smaller number of computers.