Virtual Hosting

Virtual hosting is a method for hosting multiple domain names on a computer using a single IP address. This allows one machine to share its resources, such as memory and processor cycles, to use its resources more efficiently.
One widely used application is shared web hosting. Shared web hosting prices are lower than a dedicated web server because many customers can be hosted on a single server.


Types
Name-based
Name-based virtual hosts use multiple host names for the same webserver IP address.

With web browsers that support HTTP/1.1 (as nearly all now do), upon connecting to a webserver, the browsers send the address that the user typed into their browser's address bar (the URL). The server can use this information to determine which web site, as well as page, to show the user. The browser specifies the address by setting the Host HTTP header with the host specified by the user. The Host header is required in all HTTP/1.1 requests.

IP-based
In IP-based virtual hosting each site (either a DNS hostname or a group of DNS hostnames that act the same) points to a unique IP address. The web server is configured with multiple physical network interfaces, virtual network interfaces on the same physical interface or multiple IP addresses on one interface.

The web server can obtain the address the TCP connection was intended for using a standard API and use this to determine which website to serve. The client is not involved in this process and therefore (unlike with name based virtual hosting) there are no compatibility issues.

Port-based
The default port number for HTTP is 80. However, most webservers can be configured to operate on almost any port number, provided the port number is not in use by any other program on the server.