HTTP the Definite Guide says

Before an HTTP client can send a message to a server, it needs to establish a TCP/IP connection between the client and server using Internet protocol (IP) addresses and port numbers.


In TCP, you need the IP address of the server computer and the TCP port number associated with the specific software program running on the server. This is all well and good, but how do you get the IP address and port number of the HTTP server in the first place? Why, the URL, of course! We mentioned before that URLs are the addresses for resources, so naturally enough they can provide us with the IP address for the machine that has the resource.

Where do we get URLs? From a HTTP request to be sent, or by HTTP client passing it independently of the HTTP request?



Where do you get URLs?

An human either:

  • enters the URL in the address bar of its browser, because it has seen it somewhere
  • has it in its bookmark, so just selects it
  • or is visiting a website (or the results of some searches on a search engine) and clicks on a link which is a new URL.

The URL contains an hostname (could be directly in IP address but this is rare), which will be solved to an IP address through the DNS, and as for the port number, if it is not mentioned in the URL itself, the default value is 80 for HTTP and 443 for HTTPS.

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