Ethernet - Arduino Sketches: Tools and Techniques for Programming Wizardry - page 153

MAC Address
The MAC address is the hardware address of the network connector. Each
device has its own specii c address, and in theory, no two devices should have
the same MAC address.
IP Address
This address is dei ned by the user or by the network administrator. It is the
address used to identify a network device, both for sending information and for
receiving. It is possible to have devices that use the same address, and indeed,
this happens every day. Your modem probably has a local address like 192.168.0.1,
and your neighbor might have this address, too.
IP addresses are made out of 4 bytes. Normally, the i rst 3 bytes are the net-
work, and the fourth is the machine on that network. The network 192.168.0.XXX
is an “internal” network, one that is shielded from the Internet. You can add
any devices.
DNS
Humans are good at remembering text but not so good at remembering num-
bers. When you want to connect to Wiley's Internet site to get more information
about new topics, you can enter http://www.wiley.com in to your browser. This
address, however, does not name a machine; machines can be contacted only
by their IP address. You can almost certainly remember the text www.wiley
.com, but could you remember 208.215.179.146? Probably not. To counter this,
DNS was invented. DNS, short for Domain Name Service, is a large database
that translates human readable domain names (like www.wiley.com) into the
more difi cult IP address system. All the code presented in this topic is available
on Wiley's website, and to download the code, you need to enter Wiley's web
address into your browser. Your browser might not know Wiley's IP address,
and if it doesn't, it will send a request to a DNS server. The DNS request will
say, “Hey, could you please tell me the address of www.wiley.com?” Th e DNS
server will respond with either the IP address of the request, or an error message
if it does not exist. Your browser can then contact Wiley's server.
Port
To connect to a server (a machine that will provide a service), a client (some-
thing that requires this service) requires two things: the address of the server