Lecture 2: Internet Overview

Application Protocols: The Web

A Web server process waits for connections at the well known^[2] port number 80. A Web browser (eg Netscape, IE, Opera -- the client in this transaction) initiates a connection to the Web server. Visualise what happens next by considering a URL:

The protocol section of the URL specifies HTTP, the HyperText Transfer Protocol. This protocol is associated with the well-known port number 80 -- that is, when we connect to a server process at port 80, we expect to "talk HTTP". The domain name of ironbark is just an alternative way of specifying its IP address, which is actually 149.144.21.60 -- see later for more on this. And the desired file on ironbark is (note Unix terminology) "/index.html".

In the simplest version of HTTP (HTTP/0.9 - circa 1993), the client (ie, the Web browser) sends a line of plain, ASCII text to the server process, thus:

GET /index.html

The server responds by returning the contents of the file /index.html, also in ordinary plain (ASCII) text. Finally, the browser process interprets the HTML markup in the returned file, and displays it to the user.

^[2] We shall return to the topic of well-known services later, but basically all of the server port numbers below 1024 are reserved for generally agreed services.

Lecture 2: Internet Overview

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