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IP addresses

Each device on a LAN (Local Area Network) must

have a unique address. This is commonly called the ‘IP

address’, and is occasionally referred to as the Ethernet

address. An IP address consists of four numbers separa-

ted by a dot ‘ . ’, each number is in the range 0-255. For

example, the address could be 192.36.253.80.

The ﬁ rst three groups of digits will be common to

all devices connected to the same segment, i.e. in the

previous example, all units within the same segment will

have a common address beginning with 192.36.253.

Each IP address is divided into 65,535 ports. Dif ferent

applications use different IP ports. HTTP (ordinary Web

browsing) usually uses port 80 (e.g. 192.36.253.80:80).

Normally users do not need to concern themselves with

port numbers.

Data packets

All data is sent within a data packet, and all packets

are labeled with the address of the destination. In an

Ethernet network a packet is transmitted approximately

every 0.1 milliseconds. This means that up to 10,000

packets can be transmitted every second.

As modern computers and network devices have a

high capacity, they can communicate simultaneously

with several units. A modern network camera can send

images to at least ﬁ ve computers simultaneously.

NAT routers

All devices connecting directly to the Internet must

have a unique public IP address. Public IP addresses

are sold by Internet Service Providers (ISP’s). A device

called a Network Address Translator, NAT, can separate

a LAN net from the Internet. A NAT can either be a small

box or a program running on a computer. Most ofﬁ ce

LANs are normally connected to the Internet to make it

possible to send e-mails and to browse the Internet. This

connection is normally made via a modem.

Gateways

Gateways provide a convenient way to create a local

network. A gateway works as a combined router, switch

and NAT and is available from various manufacturers,

for example Cisco, D-Link and 3Com.

A typical network set up within a small ofﬁ ce using ADSL connection to Internet. The NAT working as a router makes it possible to separate the LAN from the Internet.

Example of an Ethernet network with 14 devices. The devices can be PCs, printers, network cameras or any other piece of equipment with an Ethernet connector.

In this example the switches have 9 connectors each. Two more devices can easily be added. To add more than two, another switch would be needed.

N ETWORK BASICS

COMPANY OR INTERNET PROVIDER

CONNECTION TO A PHONE

PRINTER

SCANNER

CAMERAS

SWITCH

NAT ADSL MODEM

10.112.46.10

10.112.46.11

10.112.46.13