Operation Manual

Human-computer interfacing
120
Notes:
Lesson 4.4: Web applications
A web application is a program that communicates on the World Wide Web.
Like any other program, it has inputs, processes them and produces some output.
Not every input and output from a web application has to be through a web
browser, though!
The World Wide Web uses something called HTTP (Hyper Text Transfer Protocol).
This is what web servers and web browsers use to communicate with each other
at a low level. The port number used for HTTP is 80. The detail of how HTTP works
does not need to concern us at this stage – all the work is done for us by web
browsers and libraries in Python.
A URL (Uniform Resource Locator), otherwise known as a web address, is what
you would type into the address bar of a web browser. It can be made up of a
few parts:
protocol :// hostname : port / address
Let’s look at those parts in order:
Protocol
For a website, this is normally “HTTP:” but doesn’t have to be. Secure websites
use the encrypted “HTTPS:” protocol, and most browsers can also work with the
file transfer protocol (“FTP:”).
Hostname
This is the name of the computer, the web server, on which the website resides.
In these examples, we use the hostname “localhost”, which means the computer
on which a program resides.
Port
This is a number between 1 and 65,535, specifying the TCP port to be used for the
communication. If not specified, the default port, 80, is used.
Address
This is the address of the specific webpage that you want to view. For instance,
in the URL www.raspberrypi.org/faqs the suffix “faqs” is the address.
HTML (Hypertext Markup Language) is a language used to describe the layout of
a web page. In both of these examples here, you will notice that the web page
created is fairly basic. Imagine how you can improve this just by modifying
the HTML!
Tip...
If you want to
find out the
IP address of
your Raspberry Pi,
open the Terminal,
type the command
“ifconfig” and
press Return.
This command
does the same
job as “ipconfig”
on a MS Windows
computer.