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What is a Host Header?

For 1&1 Dedicated Server Windows

Host headers were developed due to the scarcity of IPv4 IP addresses and are used to administer multiple domains with a single IP address. When receiving a request from a browser the appropriate domain name is transmitted along with the requested file names. A single Web server is thus able to accept and process requests for different domain names. Using the domain name, each request is subsequently directed to the appropriate virtual server and thereby directed to the respective (folder) path.

Multiple Virtual Servers can be used for different domains, but the same IP address and the same port (80) are used at the same time. The higher-level Web server uses the host header to direct requests to the appropriate virtual servers. If you have applied for a domain, there should be two virtual servers with the following configuration: One for the domain you applied for and another one for the "u123.onlinehome-server.com" (or similar) domain.

The host header configuration for your server can be found in the properties of a virtual server on the "Web Site" tab via the "Advanced..." button next to the IP address.

Website Properties
Website Properties
Advanced Web Site Identification
Advanced Web Site Identification

Your domain is registered with and without "www" on the name server, so requests to http://1and1sample.co.uk and http://www.1and1sample.co.uk will both work. If the entry using "www" were missing, this virtual server would not be in charge of handling a request with the "www" heading. If no instance can be found to handle a given host header, the request goes unanswered or the web server will return a "bad request" error message.

Please note:
Incorrectly configured host headers are a frequent source of errors. Make sure that the combination of IP address, port number and host header value are always unique. The same host header in different virtual servers may result in one of the servers being unable to start.

You can also enter the IP address in a host header for testing purposes. Your server could then, for example, also be reached by using If your server is configured via DHCP, you should not expect the IP address of your server to remain permanently unchanged.

For additional information, you may want to reference: