HTTP headers / request header
Host: api.example.com
The hostname (and optional port) the request targets — mandatory in HTTP/1.1, the basis of virtual hosting and SNI routing.
Sending the wrong Host against a shared IP returns someone else's site or a 421; HTTP/2 carries it as the :authority pseudo-header.
The fastest way to see what Host is actually doing is to send the request and read both sides raw — ReqPad shows auto-generated and custom headers for every request, on all six protocols, with history. Related references: status codes · Authorization · Accept · Accept-Encoding · Accept-Language · If-None-Match · If-Match
Build the request, send it, read raw headers and timing — from your iPhone. Free to start.