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.
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.
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 http://126.96.36.199. If your server is configured via DHCP, you should not expect the IP address of your server to remain permanently unchanged.