Chapter 6: Internet Infrastructure

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Chapter 6: Internet Infrastructure

i
-
Net+ Guide to the Internet

Third Edition

iNet+ Guide to the Internet, Third Edition

2

Objectives


Learn how networks are segmented to improve
performance and how hardware devices work to
segment a network


Investigate the way large networks are divided
logically into subnets to make them easier to manage
and improve performance


Study data routing across the Internet

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Objectives (Continued)


Learn how domain names are used and managed on
the Internet


Learn how servers and Internet security appliances
can be used to enable and improve the performance
and reliability of a network and the Internet


Compare bandwidth technologies used on LANs,
WAN, and the Internet

iNet+ Guide to the Internet, Third Edition

4

Physically and Logically Dividing a Large
Network


You can divide a network using two approaches:

1.
You can physically divide the network using hardware
devices

2.
You can logically divide the network using software
settings


The first approach is called
segmenting

the
network; the second approach is called
subnetting
.

iNet+ Guide to the Internet, Third Edition

5

Bridges and Switches


Bridges

and
switches

are more intelligent than hubs
and make decisions involving whether to allow traffic
to pass or where to route that traffic, reducing traffic
on each segment and improving network
performance.


A
routing table

is a database stored within a router
that is used to find the best network path on which to
forward information.


A network bridge keeps routing tables for each
network to which it connects.

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Bridges and Switches (Continued)


The tables start out empty and all data packets that
reach the bridge from one segment are passed on to
the other segment connected to the bridge.


Just like bridges, switches keep tables of the MAC
addresses of all the devices connected to the switch.


Switches use these tables to determine which path to
use when sending packets.


However, unlike a bridge, a switch passes a packet
only to its destination segment instead of to all
segments other than the one it came from.

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Bridges and Switches (Continued)


Bridges and switches use MAC addresses to
subdivide a network into physical segments.


However, all the segments are still logically a single
network because each host is communicating with
other hosts on other segments using the MAC
address rather than the IP address.


As far as a host is concerned, it is not aware that a
bridge or a switch exists in the network.

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Subnetting


A large network can be logically divided into two or
more networks based on IP addresses rather than
MAC addresses to reduce congestion.


Each division is called a
subnet

and the process is
called subnetting.

iNet+ Guide to the Internet, Third Edition

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Subnet Masks


How does the host know if a remote host is on the
same network?


An IP address is made up of the network ID and the
host ID.


The host is told what portion of the IP address
identifies the network by an entry in the TCP/IP
configuration settings.


This entry is called the
network mask
, or
subnet
mask
, and is used to define which portion of an IP
address identifies the network and which portion
identifies the host.

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Subnet Masks (Continued)


The network mask is a group of four 8
-
bit numbers separated by
periods.

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Subnet Masks (Continued)


If the network IDs had been different, the host would
not have attempted to resolve the IP address to the
MAC address, but would have sent the data to the
gateway to its network.


A
gateway

is any device, typically a router, that
provides access to another network.


Subnet masks usually are not displayed as 32 bits
separated by periods as they are in Table 6
-
1.

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Selecting a Subnet Mask


A network engineer carefully selects a subnet mask
based on the number of subnets he needs and the
number of hosts planned for each subnet.


Table 6
-
3 on page 351 shows several examples of
subnet masks and explains the number of hosts and
subnets that can use each subnet mask.


Subnetting is necessary when a large company is
using a Class A, B, or C license for its entire network
and wants to use that one license over several
networks to prevent network congestion.

iNet+ Guide to the Internet, Third Edition

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How Data Travels Across

Interconnected Networks


Figure 6
-
12 shows a simplified view of how networks
work together to send data over the maze of many
networks called the Internet.


A router is a
stateless

device, meaning that it is
unconcerned about the data that it is routing, but it is
concerned about the destination address of that data.


Networks are connected by routers, which belong to
more than one network.

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How Data Travels Across Interconnected
Networks (Continued)

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Routers


Routers
are responsible for helping data travel
across interconnected networks.


A router can forward data to the correct network in a
way that is similar to a switch’s method.


A router uses the most efficient path available to
forward packets to their destination, which may be
located across a great geographical distance.

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Routers (Continued)


A router can transmit a data packet to a remote
network only if the higher
-
layer protocol that was
used to produce the data packet can be routed to a
remote network.


A
brouter
, short for bridge and router, functions as
both a bridge and a router.


The device can forward routable protocols, including
TCP/IP and IPX/SPX packets, and in these cases, is
working as a router.

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Routers (Continued)


Packets that are not routable, such as NetBEUI
packets, are forwarded to other local networks in the
manner that a bridge would forward packets.


Like switches, routers use tables to determine the
best route by which to send the data to its
destination.


When routers communicate with other routers to build
routing tables and determine availability of routes,
one of several protocols is used: RIP, OSPF, BGP,
DVMRP, NLSP, or IGRP.

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TCP/IP Routing


Suppose a host computer wants to send data to
another host.


Remember that the host uses its subnet mask to
decide if the destination host is on its own or another
network.


If the first host knows that the remote host is on its
same network, it must discover the MAC address of
the remote host.

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TCP/IP Routing (Continued)


If the sending host determines that the remote host is
on a different network, it sends the data to the router,
which is serving as the gateway to remote networks.


When a packet arrives at a router, the router decides
if the packet belongs to a host within its own local
network or needs to be routed to a different network.

iNet+ Guide to the Internet, Third Edition

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Routing Across Many Networks


For routing across interconnected networks, each
time a packet encounters a router, its TTL is reduced
by one.


If the router must send the packet over a network that
cannot handle large packets, the router divides the
packet into smaller packets.

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Default Gateways


Sometimes, a large network has more than one
router, as shown in Figure 6
-
21on page 362, and so
the network has more than one gateway to other
networks.


The network in the upper
-
left of the figure is 250.1.2
and has two routers (D and E), each of which also
belongs to other networks.

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Default Gateways (Continued)


Host E is designated as the
default gateway
,
meaning that hosts on the 250.1.2 network send
packets addressed to other networks to this gateway
first.


The other router Host D, is called the
alternate
gateway

and is used if communication to the default
gateway fails.

iNet+ Guide to the Internet, Third Edition

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Domain Names on the Internet


Domain names are assigned because IP address
numbers are difficult to remember and because
companies might want to change their IP addresses
without also changing the Internet name by which the
outside world knows them.


The last segment, or suffix, of a domain name is
called the
top
-
level domain

and tells you something
about the function of the host.


The first word in a domain name is used to identify a
subcategory within the domain and is called a
canonical name
, or
CNAME
.

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Assigning and Tracking Domain Names
and IP Addresses


The organization responsible for overseeing this operation
is the
IANA (Internet Assigned Numbers Authority).


Beginning in the spring of 1999, the responsibility for
assigning and tracking domain names and IP addresses
was transitioned from IANA to a nonprofit, private sector
organization regulated by the U.S. Department of
Commerce called
ICANN (Internet Corporation for
Assigned Names and Numbers)
.


A company that can register these names and numbers
must be approved by ICANN and is called a
registrar.

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Domain Name Resolution


Domain names and IP addresses do not have to be
permanently related.


Two name resolution services track relationships between
domain names and IP addresses:
DNS (Domain Name
System
, also called
Domain Name Service)

and
Microsoft
WINS (Windows Internet Naming Service)
.


DNS is the more popular of the two because it works on
all platforms.


At the heart of DNS is a distributed database, which
initially must be created manually.

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How DNS Works


DNS has three logical components:


Computers searching for the IP address for a domain
name, called
resolvers


Servers that contain the information relating domain
names to IP addresses, called
name servers


The databases of information needed to resolve
domain names and IP addresses, called
namespaces

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How DNS Works (Continued)


The process of discovering an IP address for a given
domain name is called
address resolution
.


It is also possible to find the domain name for a given
IP address; this process is called
reverse
resolution,
or
reverse mapping
.


Name servers are organized from the top down, as
shown in Figure 6
-
24 on page 367.


Network Solutions maintains servers called
root
servers

that act as the highest level of authority
when locating domain name information.

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How DNS Works (Continued)


A network that supports DNS has two or more name
servers, called the
primary name server

and
secondary name server
, which are shown in Figure
6
-
24.


The secondary server gets its information from the
primary server, and is sometimes called the
slave
name server
.


An
authoritative name server

is the server that has
the most current information about a domain name.

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How DNS Works (Continued)


The group of networks for which the name server is
responsible collectively is called the name server’s
zone
.


A zone also can have a
caching
-
only server

that
does not keep authoritative information, but only
caches information as it is used in case it is needed
again within a short period of time.

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DNS Records


Each name server holds a piece of the namespace,
which is the database containing information needed
to resolve domain names and IP addresses.


A name server keeps the entries for each domain
name that it knows about in a
resource record
, or
DNS record
.

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Directory Server


A
directory server

stores information about people,
hosts, and other resources on the network in
directories and provides this information to computers
on the network.


The information in a directory is read more often than
it is written.

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How Directories Work


Directories follow an upside
-
down tree structure with
the root at the top and branches underneath the root
in a hierarchical structure.


Directory servers sometimes use a protocol called
LDAP (Lightweight Directory Access Protocol)

to
access directories, as illustrated in Figure 6
-
28.


LDAP was designed to run over TCP and can be
used on the Internet or on an intranet.

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How Directories Work (Continued)

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How Directories Work (Continued)


LDAP is a “lighter” version of DAP (Directory Access
Protocol); LDAP has less code than DAP.


Another important directory standard is
X.500
, which
specifies how global directories can be structured.


X.500 directories are designed to provide a listing of
people within an organization so that anyone with
Internet access can look someone up by country,
organization, organizational level, or name.

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Using Directories


Directories and
directory servers can
serve various
functions on
networks and on the
Internet.


Directories on the
Web are similar to
search engines in
the way they operate
and provide
information.

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Cache Servers


Microsoft Internet Explorer supports
browser
caching
, which allows the user to indicate how much
hard drive space should be allocated to Web caching.


A
cache server

improves performance by caching
data so that the number of requests to the Internet is
reduced.

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Cache Servers (Continued)


Cache servers save Web pages and other files that
users have requested so that when a page is
requested again, it can be retrieved without
accessing the Internet.


Cache servers are placed between users and the
Internet.


A cache server can run on a system such as a proxy
server or a router, or it can be set up as a dedicated
computer system.

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Mirrored Servers


A
mirrored server

carries the same data and
services as another server. These servers are exact
replicas of the main servers that they mirror and are
updated often to ensure that they contain the same
data.


Mirrored servers have two main purposes:


They reduce download time for users by handling
some of the traffic for a frequently accessed Web site.


They serve as backups for the main server in case it
goes down.

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Using a Mirrored Server

to Handle Site Traffic


Web sites that get a lot of traffic often need more
than one server.


If the traffic comes from different parts of the world, it
might be necessary to have servers in different
locations to provide the best service for international
customers.

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Using a Mirrored Server

as a Backup Server


A mirrored server acts as a very effective backup
system.


If the main server goes down, it is faster and easier to
switch operation to a mirrored server than it is to
restore information from disks and tapes.


When a mirrored server is used as a backup for a
server, it constantly copies short segments of files
from the main server as they are updated.

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Print Servers


Print servers

make printers available for shared use
across a network or even across the Internet.


Each print server can have several printers attached
to it, and you can have more than one print server on
a network, depending on the size of the network and
the needs and locations of the users on it.

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LPD Servers


LPD (Line Printer Daemon) server

is print server
software that initially was developed on UNIX servers
to handle print jobs, but is now supported by
Windows Server 2003, Windows 2000 Professional,
and Windows NT Server.


A client communicating with an LPD server can use
two protocols, LPR and LPQ, which are part of the
TCP/IP protocol suite.

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LPD Servers (Continued)


The client uses the
LPR (Line Printer Remote)

protocol to send print jobs over a TCP/IP network to
the server.


Clients that use LPR are sometimes referred to as
LPR clients.


A second protocol,
LPQ (Line Printer Queue)
, is
required for users to be able to check on the status of
print jobs they have sent.

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IPP


Another useful and more recently developed printing
protocol is
IPP (Internet Printing Protocol)
.


IPP, which also enables printing across LANs and the
Internet, is generally more versatile than LPD and its
associated services, and is more easily compatible
with various operating systems.


The greatest benefit of IPP is being able to find a
printer by using the printer’s IP address or URL.


With IPP, you can find any Internet
-
connected printer,
print to it, and check the status of your print job.

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Internet Security Appliances


Internet security appliances
, once called an
Internet
-
in
-
a
-
box, are becoming a popular Internet
access solution for small businesses.


These devices combine a variety of technologies,
such as Internet prevention, and other networking
capabilities, into a single easy
-
to
-
manage unit.


Remote Access Service (RAS)

provides a way for a
remote user to log on to the network using telephone
lines and a modem.

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Bandwidth Technologies


Much attention is given to the amount of data that
can travel over a given communication system in a
given amount of time.


This measure of data capacity is called
bandwidth
,
also called
data throughput

or
line speed
.


The greater the bandwidth, the faster the
communication.


A list of bandwidth technologies, their speeds, and
their uses is shown in Table 6
-
6 on pages 381
through 383.

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Bandwidth Technologies Used

to Connect to an ISP


A local area network has much less need for data throughput
than does a national backbone.


Lying between these two extremes on the spectrum are many
types of systems that require varying degrees of bandwidth.

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Regular Telephone Lines


Regular telephone lines, the most common way to
connect to an ISP, require an internal or external
modem.


When data packets travel over telephone lines, the
Data Link layer protocol used is PPP or SLIP.


PPP (Point
-
to
-
Point Protocol)

most often is used to
transmit TCP/IP packets from a computer connected
to an ISP or intranet access point by telephone line.

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Cable Modem


Cable modem

communication uses cable lines that
already exist in millions of households in the United
States.


Just as with cable TV, cable modems are always
connected.


A cable modem is an example of broadband media.


Broadband

refers to any type of networking media
that carries more than one type of transmission.

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PPPoE (Point
-
to
-
Point Protocol over
Ethernet)


PPoE (Point
-
to
-
Point Protocol over Ethernet)

is a
protocol that adapts PPP to work with Ethernet.


PPPoE describes how the computer is to interact with the
converter box or modem when the two are connected by
an Ethernet cable connected to an Ethernet network card
in the computer.


PPPoE gives the user the security and authentication that
is offered with PPP.


PPPoE also sets standards for networks to connect to the
Internet via DSL modems and other high
-
speed access
services.

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ISDN


ISDN (Integrated Services Digital Network)

is a
technology developed in the 1980s that uses regular
telephone lines, and is accessed by a dial
-
up
connection.


ISDN is actually an early implementation of DSL.

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DSL


In the race to produce a fast data transmission
technology that is affordable for home use and that
offers a direct connection rather than a dial
-
up
connection, the telephone industry has developed
several similar technologies that collectively are
called
DSL (Digital Subscriber Line)
.


Table 6
-
7 on page 390 lists common variations of
DSL.


The most popular version of DSL is
ADSL
(Asymmetric Digital Subscriber Line)
, which is 50
times faster than ISDN and is direct connect.

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Satellite Connections to the Internet


People who live in remote areas and want high
-
speed
Internet connections often are limited in their choices.


DSL and cable modems might not work where they live,
but satellite access is available from almost anywhere.


Technology is even being developed to use satellites to
offer Internet access on commercial airlines.


New technology allows data to be transmitted both ways
over the satellite so that telephone line connections are
not needed.

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Wireless Connections


Wireless is an important technology for mobile
devices and for Internet access in remote locations
where other methods are not an option.


For Internet access, two popular applications of
wireless are fixed
-
point wireless, sometimes called
Wireless Local Loop (WLL), and mobile wireless.


With fixed
-
point wireless, an antenna sits on your
house or office building and communicates with a
base station antenna.

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Using Wireless Technology

to Browse the Internet


Most of the wireless devices that advertise Internet access
are menu
-
driven, which means that you select where you
want to go from a menu instead of typing in a URL.


After the browser has been launched, you might be
charged for the time you are connected.


Most Web sites still are not designed with wireless
technology in mind, and some devices run software that
converts HTML so it can be displayed on wireless devices.


This conversion process is called
clipping

because it
takes out, or clips, the images and leaves a site with all
text that uses menus to navigate.

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Wireless Application Protocol


The high demand for wireless access to the Internet,
including Web pages and e
-
mail, led to the creation
of the
WAP (Wireless Application Protocol)
.


One goal of WAP is to bridge the gap between the
needs of traditional Internet access devices and
wireless Internet access devices.


WAP is a communication standard designed for
mobile Internet access.


One feature of WAP is
WML (Wireless Markup
Language)
.

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Wireless Application Protocol (Continued)


WML is a markup language that is very similar to
HTML, but it is derived from
XML (Extensible
Markup Language)
.


WML files are called
decks

and are divided into
cards.


Cards

are sections of the deck that fit onto one
screen.

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Uses of Wireless Devices


One popular service that is being provided to
customers with wireless Internet devices is instant
notification, or alerts.


If you were invested heavily in a particular stock, for
example, wouldn’t it be nice to be notified
immediately if the value changed?

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T Lines and E Lines


The first successful system that supported digitized
voiced transmission was introduced in the 1960s and
was called a
T
-
carrier.


A T
-
carrier works with a leased digital
communications line provided through a common
carrier, such as Bellsouth or AT&T.


The leased lines are permanent connections that use
multiplexing
, a process of dividing a single channel
into multiple channels that can be used to carry
voice, data, video, or other signals.

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T Lines and E Lines (Continued)


The E
-
carrier is the European equivalent of the
American T
-
carrier.


The
E
-
carrier

is a digital transmission format devised
by ITU at
www.itu.int
.


A
fractional T1 line

is an option for organizations
that don’t need a full T1 line.


The fractional T1 allows businesses to lease some of
the channels of a T1 line rather than leasing all 24
channels.

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X.25 and Frame Relay


Both
X.25

and
frame relay

are packet
-
switching
communication protocols designed for long
-
distance data
transmission rather than the circuit
-
switching technology
used by the telephone system.


Both X.25 and frame relay use a
PVC (permanent virtual
circuit)
.


PVC is a permanent, logical connection between two
nodes.


PVCs are not dedicated lines, like the T
-
carriers.


The biggest advantage of X.25 and frame relay is that you
only have to pay for the amount of bandwidth you require.

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ATM


ATM (Asynchronous Transfer Mode)

is a very fast network
technology that can be used with LANs, as well as WANs.


It uses fixed
-
length packets, called
cells
, to transmit data, voice,
video, and frame relay traffic.


ATMs also use virtual circuits, meaning that the two endpoints
are stationary, but the paths between these two endpoints can
change.


They can use either PVCs or SVCs.


SVCs (switched virtual circuits)

are logical, point
-
to
-
point
connections that depend on the ATM to decide the best path to
send the data.

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Mesh Topology


A
mesh topology

provides multiple point
-
to
-
point
links between routers in a wide area network, giving
more than one choice on how data can travel from
router to router.


In a mesh topology, a router searches out multiple
paths and determines the best path to take.

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Summary


To relieve congestion, a network can be segmented
into smaller networks by using a bridge, switch, or
router.


A large network logically can be divided into subnets
by using a subnet mask, which takes a few bits from
the network portion of the IP addresses on a network
to define the subnets on the network.

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Summary (Continued)


Domain names are an easy way to remember an IP
address, but also can be assigned to different folders
on a host so that a host can have many domain
names assigned to it.


Web caching can be used to store frequently used
Web pages in a temporary place to decrease
download time.


Packet
-
switching divides data into packets and sends
each packet independently.