Hardware Reference
In-Depth Information
Network Identification
So far, you've identified network devices computationally by their address. For devices
on the Internet, you've seen both IP and MAC addresses. Bluetooth and 802.15.4
devices have standard addresses as well. However, the address of a device doesn't tell
you anything about what the device is or what it does.
Recall the networked air-quality project in Chapter 4.
The microcontroller made a request via HTTP and the
PHP script sent back a response. Because you already
knew the microcontroller's capabilities, you could send
a response that was short enough for it to process effi-
ciently, and format it in a way that made it easy to read.
But what if that same PHP script had to respond to HTTP
requests from an Arduino Ethernet, a desktop browser
like Safari or Chrome, and a mobile phone browser? How
would it know how to format the information?
messages—about the sender's and receiver's identity and
capabilities. You can use these to your advantage when
designing network systems like the ones you've seen
here. There's not space to discuss this concept
comprehensively, but following are two examples that
use HTTP and mail.
HTTP Environment Variables
When a server-side program, such as a PHP script,
receives an HTTP request, it has access to a lot more
information about the server, the client, and more than
you've seen thus far.
Most Internet communications protocols include a basic
exchange of information—as part of the initial header
To see some of it, save the
following PHP script to your web
server, then open it in a browser.
Name it env.php.
8
<?php
/*
Environment Variable Printer
Context: PHP
Prints out the environment variables
*/
foreach ($_REQUEST as $key => $value)
{
echo "$key: $value<br>\n";
}
foreach ($_SERVER as $key => $value)
{
echo "$key: $value<br>\n";
}
?>
