When ftp was first concieved it was only possible to host a single ftp server on any given box. A method to increase the hosting density from one site per server to many sites on a given server grew. This many to one mapping is Virtual hosting. The design change in the server software was to allow for multiple unique ftp server configurations and binding these to particular interfaces on the server. Densities of hundreds of ftp sites per serving machine are not unknown on today's Internet.