Virtual Hosting

Virtual hosting is a method for hosting multiple domain names (with separate handling of each name) on a single server (or pool of servers). This allows one server to share its resources, such as memory and processor cycles, without requiring all services provided to use the same host name. The term virtual hosting is usually used in reference to web servers but the principles carry over to other internet services.

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. It is also very common for a single entity to want to use multiple names on the same machine so that the names can reflect services offered rather than where those services happen to be hosted.

There are two main types of virtual hosting, name based and IP based. Name based virtual hosting uses the host name presented by the client. This saves IP addresses and the associated administrative overhead but the protocol being served must supply the host name at an appropriate point. In particular there are significant difficulties using name based virtual hosting with SSL/TLS. IP based virtual hosting uses a separate IP address for each host name, and it can be performed with any protocol but requires a dedicated IP address per domain name served. Port based virtual hosting is also possible in principle but is rarely used in practice because it is unfriendly to users.

Name based and IP based virtual hosting can be combined, a server may have multiple IP address and serve multiple names on some or all of those IP addresses. This technique can be useful when using SSL/TLS with wildcard certificates. For example if a server operator had two certificates one for *.example.com and one for *.example.net he could serve foo.example.com and bar.example.com off the same IP address but would need a separate IP address for baz.example.net.