Virtual hosting is a method that web servers use to host more than one domain name on the same computer and IP address.
With web browsers that support HTTP/1.1 (as most do), upon connecting to a webserver, they send the address that the user typed into their browser's address bar (the URL). The server can use this information to determine which webpage to show the user.
For instance, a server could be receiving requests for two domains, www.site1.com and www.site2.com, both of which resolve to the same IP address. For www.site1.com, the server would send the HTML file file from the directory /www/JoeUser/site1/, while requests for www.site2.com would make the server serve pages from /www/FrankUser/site2/.
Pros
Virtual hosting makes it much more cost-effective to serve many different websites, since only one computer is required, and, in many cases, only one set of web-serving programs needs to be running on the system. However, there are a few situations where the arrangement breaks down.
Cons
If the Domain Name System (DNS) is not properly functioning, it becomes much harder to access a virtually-hosted website. Ordinarily, in this case, the user could try and fall back to using the IP address to contact the system, as in http://12.34.56.78/. However, the web browser doesn't know what hostname to send when this happens, so the server will respond with a default website—often not the site the user expects.
A workaround in this case is to add the IP address and hostname to the client system's hosts file. At this point, accessing the server with the domain name should work again. However, users should be careful when doing this, as any changes to the true mapping between hostname and IP address will be overridden by the local setting.
Another issue with virtual hosting is the inability to host multiple secure websites running Secure Sockets Layer or SSL. Because the SSL handshake takes place before the expected hostname is sent to the server, the server doesn't know which encryption key to use when the connection is made. One workaround is to run multiple web server programs, each listening to a different incoming port, which still allows the system to just use a single IP address. Another option is to do IP aliasing, where a single computer listens on more than one IP address.
Resources about Virtual Hosting
