Strange error message

Unable to access a number of websites

[vibgyor79]

I am unable to access a number of websites. I keep getting this error message since morining -

Host: header containing unknown local host name

There are four possible reasons you are seeing this error message.

1) The domain you are trying to view was added to our server cluster since the last time the server cluster synchronized with the main server. Our server cluster synchronizes once each day. If this is the reason you are seeing this error message, please come back to this URL 12-24 hours from now. This error message will most likely be replaced with the correct website by then.

2) You may have mistyped the URL.

3) You are using a really old browser, such as Netscape 1.0. All of the sites we host use hostname based multihoming. If you are using Netscape 1.0 or IE2, please upgrade to a newer version of your favorite browser. Netscape 2.02 and newer will work. MS-IE3 and newer will work.

4) You typed the IP address of our load balancer into your browser instead of the domain name of your site.

Any idea what this means? What is a load balancer?

[vibgyor79]

Added: Got this error message for sites found through Google. That is, clicking on Google search results for some of the sites give this error message

[universalis]

The sites in question are hosted using name-based virtual hosting - that is, several web sites sharing the same IP address. The server reads the domain name you used to access the IP address, and supplies the site aliased to that domain name. This is a very common setup for smaller sites - for example I use a hosting company which has more than 30,000 sites all sharing the same IP address.

A load balancer is used when there are several physical servers replying to requests - the load balancer distributes incoming requests to each server and makes sure each machine is not overloaded. For example, Google runs a few thousand machines, all replying to requests, and it is their load balancer which selects which machine in the server farm replies.

Getting back to your problem. It can either be related to a configuration problem with the hosting company - normally because the domain name with which you connected is no longer aliased with a virtual server on their system, or it is because you are requesting the pages using HTTP 1.0 rather than HTTP 1.1. All modern browsers (and most old ones too!) can use 1.1, but you may have set a preference to use 1.0 by accident. Also, some older proxy servers only use 1.0.