Copyright © 2004-2005, Vivato, Inc.
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NAT
Network Address Translation
is an Internet standard that masks the internal IP addresses being used in a
. A NAT server running on a gateway maintains a translation table that maps all internal IP addresses
in outbound requests to its own address and converts all inbound requests to the correct internal host.
NAT serves three main purposes: it provides security by obscurity by hiding internal IP addresses, enables
the use of a wide range of internal IP addresses without fear of conflict with the addresses used by other
organizations, and it allows the use of a single Internet connection.
Network Address
See
.
NIC
A
Network Interface Card
is an adapter or expansion board inserted into a computer to provide a physical
connection to a network. Most NICs are designed for a particular type of network, protocol, and media, for
example,
or wireless.
NTP
The
Network Time Protocol
assures accurate synchronization of the system clocks in a network of
computers. NTP servers transmit
Coordinated Universal Time
(UTC, also known as
Greenwich Mean
Time
) to their client systems. An NTP client sends periodic time requests to servers, using the returned
time stamp to adjust its clock.
OSI
The
Open Systems Interconnection
(OSI) reference model is a framework for network design. The OSI
model consists of seven layers:
•
Layer 1, the Physical layer, identifies the physical medium used for communication between nodes. In
the case of wireless networks, the physical medium is air, and radio frequency (RF) waves are a com-
ponents of the physical layer.
•
Layer 2, the Data-Link layer, defines how data for transmission will be structured and formatted, along
with low-level protocols for communication and addressing. For example, protocols such as
and components like
addresses, and
s are all defined and dealt with as a part
of the Data-Link layer.
•
Layer 3, the Network layer, defines the how to determine the best path for information traversing the
network.
es operate on the network layer.
•
Layer 4, the Transport layer, defines connection oriented protocols such as
and
.
•
Layer 5, the Session layer, defines protocols for initiating, maintaining, and ending communication and
transactions across the network. Some common examples of protocols that operate on this layer are
network file system (NFS) and structured query language (SQL). Also part of this layer are communi-
cation flows like single mode (device sends information bulk), half-duplex mode (devices take turns
transmitting information in bulk), and full-duplex mode (interactive, where devices transmit and receive
simultaneously).
•
Layer 6, the Presentation layer, defines how information is presented to the application. It includes
meta-information about how to encrypt/decrypt and compress/decompress the data. JPEG and TIFF
file formats are examples of protocols at this layer.
