If you use multiple IP addresses, you can register each with a DNS server, allowing multiple companies or departments to use one system running multiple Web servers. Many sites that you have accessed on the Internet are served from only one of many virtual servers residing on a UNIX or NT system.
For example, IIS 4.0 and Apache Server are designed to support multiple Web server scenarios ranging from simple Web sites on a corporate intranet to large ISP Web farms. Strictly speaking, all Web servers are considered virtual; even if you are running only one Web server, IIS and Apache treat it as the primary virtual server.