The Host

The Host specifies which machine or site is being addressed. This can be a domain name or IP address, such as ShofarNexus.com or 10.20.30.40.

The low level communications on the Internet is done solely by IP addresses, a 32 or 128 bit number that specifies which machine is the source and destination of a packet.

Since people use names and not numbers, the domain name system exists that translates a name to a number.

With HTTP version 1.0 there was only one Site at an IP address. Therefore a machine could only serve one site. HTTP 1.1 added a Host record in the header which allows the Query to select one of possibly multiple sites at a single IP address.