Graphics Programs Reference
In-Depth Information
communications. At the bottom, the physical layer (1) is simply the wire and
the protocol used to send bits from one device to another. A single HTTP
message will be wrapped in multiple layers as it is passed through different
aspects of communication.
This process can be thought of as an intricate interoffice bureaucracy,
reminiscent of the movie Brazil . At each layer, there is a highly specialized
receptionist who only understands the language and protocol of that layer.
As data packets are transmitted, each receptionist performs the necessary
duties of her particular layer, puts the packet in an interoffice envelope,
writes the header on the outside, and passes it on to the receptionist at the
next layer below. That receptionist, in turn, performs the necessary duties
of his layer, puts the entire envelope in another envelope, writes the header
on the outside, and passes it on. Network traffic is a chattering bureaucracy
of servers, clients, and peer-to-peer connections. At the higher layers, the
traffic could be financial data, email, or basically anything. Regardless of
what the packets contain, the protocols used at the lower layers to move the
data from point A to point B are usually the same. Once you understand the
office bureaucracy of these common lower layer protocols, you can peek
inside envelopes in transit, and even falsify documents to manipulate the
system.
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Data-Link Layer
The lowest visible layer is the data-link layer. Returning to the receptionist
and bureaucracy analogy, if the physical layer below is thought of as inter-
office mail carts and the network layer above as a worldwide postal system,
the data-link layer is the system of interoffice mail. This layer provides a way
to address and send messages to anyone else in the office, as well as to figure
out who's in the office.
Ethernet exists on this layer, providing a standard addressing system
for all Ethernet devices. These addresses are known as Media Access Con-
trol (MAC) addresses. Every Ethernet device is assigned a globally unique
address consisting of six bytes, usually written in hexadecimal in the form
xx:xx:xx:xx:xx:xx . These addresses are also sometimes referred to as hardware
addresses, since each address is unique to a piece of hardware and is stored in
the device's integrated circuit memory. MAC addresses can be thought of as
Social Security numbers for hardware, since each piece of hardware is
supposed to have a unique MAC address.
An Ethernet header is 14 bytes in size and contains the source and destin-
ation MAC addresses for this Ethernet packet. Ethernet addressing also pro-
vides a special broadcast address, consisting of all binary 1's ( ff:ff:ff:ff:ff:ff ).
Any Ethernet packet sent to this address will be sent to all the connected
devices.
The MAC address of a network device isn't meant to change, but its
IP address may change regularly. The concept of IP addresses doesn't exist
at this level, only hardware addresses do, so a method is needed to correlate
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