HTTP Host Header
Host HTTP header holds the domain name of the web server (for virtual hosting) and the TCP port number on which the web server is listening. The port number may be omitted if the port is the standard port for the service requested. The
Host header was made mandatory in HTTP version HTTP/1.1. Although domain names are specified as case-insensitive, it is not specified whether the contents of the
Host field should be interpreted as case-insensitive. In practice, some implementations of virtual hosting interpret the contents of the
Host field as case-sensitive.
Here is an example: