Networking hard copy (1)

COMPUTER
NETWORKING

By,
SAVIN SHETTY
BHARAT SHETTY
ANKITA SHETTY
POOJA SHETTY
NIKITA RAI

INDEX

TOPICS COVERED BY
 INTRODUCTION SAVIN SHETTY

 TYPES OF TOPOLOGY ANKITA SHETTY

 TYPES OF SERVES NIKITA RAI

 TYPES OF NETWORKS BHARAT SHETTY

 COMPONENTS OF POOJA SHETTY
NETWORKS
 OSI MODEL SAVIN SHETTY

 BIBLIOGRAPHY -

What is Computer networking?

Networking is the word basically relating to computers and their connectivity. It
is very often used in the world of computers and their use in different
connections. The term networking implies the link between two or more
computers and their devices, with the soul purpose of sharing the data stored
in the computers, with each other. The networks between the computing
devices are very common these days due to the launch of various hardware and
computer software which aid in making the activity much more convenient to
build and use. Networking is the word basically relating to computers and their
connectivity. It is very often used in the world of computers and their use in
different connections. The term networking implies the link between two or
more computers and their devices, with the soul purpose of sharing the data
stored in the computers, with each other. The networks between the computing
build and use.

Computer networking is then categorized into several different areas and uses,
such as the most common ones like LAN and WAN.

Computer networking is also based on different network designs. The two basic
classification categories of the network design are the client-server and peer-to-
peer. The client-server networking refers to the computer servers that are
centralized, which are mainly used in storing emails, web pages, files and
applications. The peer-to-peer network is the most commonly used and all the
computers mainly support its functions. The Client server is used extensively
in the business functions, whereas the peer-to-peer server is for home use.

Every network requires a topology to work through which the data flows and
the computers can communicate with each other. The most common types of
topologies are bus, star, ring, and mesh.

Networking also involves a special communication language used by the
computer devices. These languages are called network protocols, and most of
the computers use a range of protocols which they support. The most common
network in the Internet and home networks is the TCP/IP.

Networking can be either wired or wireless. The most common wired networks
like Ethernet cables were extensively used but now wireless networking have
emerged and the new computer networks mainly support this feature.

With the increasing use of the computers and the networking the local area
network of the LAN is one such network type which links the two computers in
a connection.

For this connection a Local area network card or the LAN card is required
which enables the connection of the computers in a network. It is a piece of
hardware which is connected inside the PC linking the computer network.

What is Network Topology?

Computer network topology is the way various components of a network (like
nodes, links, peripherals, etc) are arranged. Network topologies define the
layout, virtual shape or structure of network, not only physically but also
logically. The way in which different systems and nodes are connected and
communicate with each other is determined by topology of the network.
Topology can be physical or logical.Physical Topology is the physical layout
of nodes, workstations and cables in the network; while logical topology is the
way information flows between different components.

Types of Physical Network Topologies

Bus Topology

Star Topology

Ring Topology

Mesh Topology

BUS TOPOLOGY :

Bus Topology is the simplest of network topologies. In this type of topology, all
the nodes (computers as well as servers) are connected to the single cable
(called bus), by the help of interface connectors. This central cable is the
backbone of the network and is known as Bus (thus the name). Every
workstation communicates with the other device through this Bus.

A signal from the source is broadcasted and it travels to all workstations
connected to bus cable. Although the message is broadcasted but only the
intended recipient, whose MAC address or IP address matches, accepts it. If the
MAC /IP address of machine doesn’t match with the intended address,
machine discards the signal.

A terminator is added at ends of the central cable, to prevent bouncing of
signals. A barrel connector can be used to extend it.

Advantages of Linear Bus Topology :

1) It is easy to set-up and extend bus network.

2) Cable length required for this topology is the least compared to other
networks.

3) Bus topology costs very less.

4) Linear Bus network is mostly used in small networks. Good for LAN.

Disadvantages of Linear Bus Topology :

1) There is a limit on central cable length and number of nodes that
can be connected.

2) Dependency on central cable in this topology has its
disadvantages. If the main cable (i.e. bus ) encounters some
problem, whole network breaks down.

3) Proper termination is required to dump signals. Use of terminators
is must.

4) It is difficult to detect and troubleshoot fault at individual station.

5) Maintenance costs can go higher with time.

6) Efficiency of Bus network reduces as the number of devices
connected to it increases.

7) It is not suitable for networks with heavy traffic.

8) Security is very low because all the computers receive the sent
signal from the source.

STAR TOPOLOGY:

In Star topology, all the components of network are connected to the central
device called nodes were connected to central cable, here all the workstations
are connected to central device with a point-to-point connection. So it can be
said that every computer is indirectly connected to every other node by the help
of “hub”.

All the data on the star topology passes through the central device before
reaching the intended destination. Hub acts as a junction to connect different
nodes present in Star Network, and at the same time it manages and controls
whole of the network. Depending on which central device is used, “hub” can act
as repeater or signal booster. Central device can also communicate with other
hubs of different network. Unshielded Twisted Pair (UTP) Ethernet cable is
used to connect workstations to central node.

Advantages of Star Topology :

1) As compared to Bus topology it gives far much better performance,
signals don’t necessarily get transmitted to all the workstations. A
sent signal reaches the intended destination after passing through no
more than 3-4 devices and 2-3 links. Performance of the network is
dependent on the capacity of central hub.

2) Easy to connect new nodes or devices. In star topology new nodes can
be added easily without affecting rest of the network. Similarly
components can also be removed easily

3) Centralized management. It helps in monitoring the network.

4) Failure of one node or link doesn’t affect the rest of network. At the
same time it’s easy to detect the failure and troubleshoot it.

Disadvantages of Star Topology :

1) Too much dependency on central device has its own drawbacks. If it
fails whole network goes down.

2) The use of hub, a router or a switch as central device increases the
overall cost of the network.

3) Performance and as well number of nodes which can be added in
such topology is depended on capacity of central device.

RING TOPOLOGY:

In Ring Topology, all the nodes are connected to each-other in such a way that
they make a closed loop. Each workstation is connected to two other
components on either side, and it communicates with these two adjacent
neighbors. Data travels around the network, in one direction. Sending and
receiving of data takes place by the help of TOKEN.

Token Passing: Token contains a piece of information which along with data is
sent by the source computer. This token then passes to next node, which
checks if the signal is intended to it. If yes, it receives it and passes the empty
to into the network, otherwise passes token along with the data to next node.
This process continues until the signal reaches its intended destination.
The nodes with token are the ones only allowed to send data. Other nodes have
to wait for an empty token to reach them. This network is usually found in
offices, schools and small buildings.

Advantages of Ring Topology :

1) This type of network topology is very organized. Each node gets to
send the data when it receives an empty token. This helps to reduces
chances of collision. Also in ring topology all the traffic flows in only one
direction at very high speed.

2) Even when the load on the network increases, its performance is
better than that of Bus topology.

3) There is no need for network server to control the connectivity
between workstations.

4) Additional components do not affect the performance of network.

5) Each computer has equal access to resources.

Disadvantages of Ring Topology:

1) Each packet of data must pass through all the computers between
source and destination. This makes it slower than Star topology.

2) If one workstation or port goes down, the entire network gets
affected.

3) Network is highly dependent on the wire which connects different
components.

4) Multistation Access Unit, and network cards are expensive as
compared to Ethernet cards and hubs.

MESH TOPOLOGY:

In a mesh network topology, each of the network node, computer and other
devices, are interconnected with one another. Every node not only sends its
own signals but also relays data from other nodes. In fact a true mesh topology
is the one where every node is connected to every other node in the network.
This type of topology is very expensive as there are many redundant
connections, thus it is not mostly used in computer networks. It is commonly
used in wireless networks. Flooding or routing technique is used in mesh
topology.

Advantages of Mesh topology:

1) Data can be transmitted from different devices simultaneously. This
topology can withstand high traffic.

2) Even if one of the components fails there is always an alternative
present,So data transfer doesn’t get affected.

3)Expansion and modification in topology can be done without
disrupting other nodes.

Disadvantages of Mesh topology

1) There are high chances of redundancy in many of the network
connections.

2) Overall cost of this network is way too high as compared to other
network topologies.

3) Set-up and maintenance of this topology is very difficult. Even
administration of the network is tough.

SERVERS

What are Servers?

Server is a computer or a device that manages network resources such
as file, printer, and user group as well as network traffic on the network.
During the server selection two parameter are very important to consider
that is server specification and processor specification.
There are also some more specification to be consider are memory,
storage, connectivity, operating system support specifications.

Servers in a data center. Several servers are mounted on a rack and connected
to a KVM switch.

Servers often provide essential services across a network, either to private
users inside a large organization or to public users via the Internet.

Types of servers

In a general network environment the following types of servers may be found.

1.)Application Server:

An application server is a server that provides software applications with
services such as security, data services, transaction support, load balancing,
and management of large distributed systems.

The term is often used for web servers that support the Java Platform,
Enterprise Edition; however its use isn't restricted to Java.

2.) Catalog Server:

A catalog server provides a single point of access that allows users to
centrally search for information across a distributed network.
In other words, it indexes databases, files and information across large
network and allows keywords, Boolean and other searches.

3.) Communications servers:

Communications servers are open, standards-based computing systems that
operate as a carrier-grade common platform for a wide range of
communications applications and allow equipment providers to add value at
many levels of the system architecture.

4.) Database servers:

A database server is a computer program that provides database services to
other computer programs or computers, as defined by the client–servermodel.
The term may also refer to a computer dedicated to running such a program.
Database management systems frequently provide database server
functionality.

Such a server is accessed either through a "front end" running on the user’s
computer which displays requested data or the "back end" which runs on the
server and handles tasks such as data analysis and storage.

5.) Fax Server:

A fax server is a system installed in a local area network (LAN) server that
allows computer users whose computers are attached to the LAN to send and
receive fax messages.

Alternatively the term fax server is sometimes used to describe a program that
enables a computer to send and receive fax messages, set of software running
on a server computer which is equipped with one or more fax-capable modems
(or dedicated fax boards) attached to telephone lines or, more recently, software
modem emulators which use T.38 ("Fax over IP") technology to transmit the
signal over an IP network. Its function is to accept documents from users,
convert them into faxes, and transmit them, as well as to receive fax calls and
either store the incoming documents or pass them on to users. Users may
communicate with the server in several ways, through either a local network or
the Internet. In a big organization with heavy fax traffic, the computer hosting
the fax server may be dedicated to that function, in which case the computer
itself may also be known as a fax server.

6.) File Server:

In computing, a file server is a computer attached to a network that has the
primary purpose of providing a location for shared disk access, i.e. shared
storage of computer files (such as documents, sound files, photographs,
movies, images, databases, etc.) that can be accessed by the workstations that
are attached to the same computer network. The term server highlights the role
of the machine in the client–server scheme, where the clients are the
workstations using the storage. A file server is not intended to perform
computational tasks, and does not run programs on behalf of its clients.

It is designed primarily to enable the storage and retrieval of data while the
computation is carried out by the workstations.

File servers are commonly found in schools and offices, where users use a LAN
to connect their client computers.

7.) Game Server:

A game server (sometimes host or shard) is a server which is the authoritative
source of events in a multiplayer video game. The server transmits enough data
about its internal state to allow its connected clients to maintain their own
accurate version of the game world for display to players. They also receive and
process each player's input.

8.) Home Server:

A home server is a server located in a private residence providing services
to other devices inside and/or outside the household through a home
network and/or the Internet.
Such services may include file and/or printer serving, media center
serving, web serving (on the network or internet), web caching, account
authentication and backup services.
Because of the relatively low number of computers on a typical home
network, a home server commonly does not require significant computing
power.

9. Print server:

A print server, or printer server, is a device that connects printers to client
computers over a network. It accepts print jobs from the computers and sends
the jobs to the appropriate printers.

Print servers may support a variety of industry-standard or proprietary printing
protocols including Internet Printing Protocol, Line Printer Daemon protocol,
Microsoft Network Printing protocol, NetWare, NetBIOS/NetBEUI, or Jet Direct.

A print server may be a networked computer with one or more shared printers.
Alternatively a print server may be a dedicated device on the network, with
connections to the LAN and one or more printers. Dedicated server appliances
tend to be fairly simple in both configuration and features. Print server
functionality may be integrated with other devices such as a wireless router, a
firewall, or both. A printer may have a built-in print server.

A wireless print server

10.) Proxy Server:

A proxy server is a server (a computer system or an application) that acts as
an intermediary for requests from clients seeking resources from other servers.
A client connects to the proxy server, requesting some service, such as a file,
connection, web page, or other resource available from a different server and
the proxy server evaluates the request as a way to simplify and control their
complexity. Today, most proxies are web proxies, facilitating access to content
on the World Wide Web.

Communication between two computers (shown in grey) connected through a
third computer (shown in red) acting as a proxy.

TYPES OF NETWORKS:
A computer network, or simply a network, is a collection of computers and
other hardware components interconnected by communication channels that
allow sharing of resources and information.[1] Where at least one process in one
device is able to send/receive data to/from at least one process residing in a
remote device, then the two devices are said to be in a network. Simply, more
than one computer interconnected through a communication medium for
information interchange is called a computer network.

Networks may be classified according to a wide variety of characteristics, such
as the medium used to transport the data, communications protocol used,
scale, topology, and organizational scope.

A local area network (LAN) is a network that connects computers and devices
in a limited geographical area such as home, school, computer laboratory,
office building, or closely positioned group of buildings. Each computer or
device on the network is a node. Current wired LANs are most likely to be
based on Ethernet technology, although new standards like ITU-T G.hn also
provide a way to create a wired LAN using existing home wires (coaxial cables,
phone lines and power lines).

Typical library network, in a branching tree topology and controlled access to
resources

A sample LAN is depicted in the accompanying diagram. All interconnected
devices must understand the network layer (layer 3), because they are handling
multiple subnets (the different colors). Those inside the library, which have
only 10/100 Mbit/s Ethernet connections to the user device and a Gigabit
Ethernet connection to the central router, could be called "layer 3 switches"
because they only have Ethernet interfaces and must understand IP. It would
be more correct to call them access routers, where the router at the top is a
distribution router that connects to the Internet and academic networks'
customer access routers.

The defining characteristics of LANs, in contrast to WANs (Wide Area
Networks), include their higher data transfer rates, smaller geographic range,
and no need for leased telecommunication lines. Current Ethernet or
other IEEE 802.3 LAN technologies operate at data transfer rates up to 10
Gbit/s. IEEE has projects investigating the standardization of 40 and 100
Gbit/s.[14] LANs can be connected to Wide area network by using routers.
A LAN is a computer network that spans a relatively small area. Most LANs are
confined to a single building or group of buildings, however, one LAN can be
connected to other LANs over any distance via telephone lines and radio waves.
A system of LANs connected in this way is called a wide-area network (WAN).
Most LANs connect workstations and personal computers.
Each node(individual computer ) in a LAN has its own CPU with which
it executes programs, but it also is able to access data and devices anywhere
on the LAN. This means that many users can share expensive devices, such as
laser, as well as data. Users can also use the LAN to communicate with each
other, by sending e-mail or engaging in chat sessions.
LANs are capable of transmitting data at very fast rates, much faster than data
can be transmitted over a telephone line; but the distances are limited, and
there is also a limit on the number of computers that can be attached to a
single LAN.

A local area network (LAN) supplies networking capability to a group of
computers in close proximity to each other such as in an office building, a
school, or a home. A LAN is useful for sharing resources like files, printers,
games or other applications. A LAN in turn often connects to other LANs, and
to the Internet or other WAN.

Most local area networks are built with relatively inexpensive hardware such
as Ethernet cables, network adapters, and hubs. Wireless LAN and other more
advanced LAN hardware options also exist.

FEATURES OF LAN

A LAN is designed for a small area. Generally it spans a single office, work
group floor in a building, or in a campus etc. LAN uses different protocols or
rules lor information transmission.

Limited No. of Users: - Most LAN supports I number of users usually around
five or ten. More users can be supported by connecting different LANs together,
which gives better results than making one; by network of the nature of MAN.

Reliability & Stability: - LANS tend to be very reliable failures on a LAN are
mostly due to wrong or improper installation and monitoring. Software that
comes along with a LAN provides a number of useful programs like error-
detection, prevention of transmission loss and excellent security features.

Flexibility: - Major development in LANs today is flexibility they offer. Earlier
versions would support only one type of desktop computers. Today's advanced
LANs however can support different types of computers. The flexibility also
extends to operating systems & storage media.

(a)Advantages of LANs

1. expensive hardware can be shared e.g. laser printer
2. network software is cheaper than buying individual packages
3. users can access the same files
4. messages can be sent between users
5. a single Internet connection can be shared among many users

(b) Disadvantages of LANs

1. quite expensive to set up and maintain
2. a virus can spread to all the computers on the network
3. more prone to hacking because of multiple points of access
4. if the file server goes down, the entire network may go down (star
network)

MAN :

A metropolitan area network (MAN) is a computer network that usually
spans a city or a large campus. A MAN usually interconnects a number of local
area networks (LANs) using a high-capacity backbone technology, such as
fiber-optical links, and provides up-link services to wide area networks (or
WAN) and the Internet.
The IEEE 802-2002 standard describes a MAN as being:[1]

“ A MAN is optimized for a larger geographical area than a LAN, ranging
from several blocks of buildings to entire cities. MANs can also depend on
communications channels of moderate-to-high data rates. A MAN might
be owned and operated by a single organization, but it usually will be used
by many individuals and organizations. MANs might also be owned and
operated as public utilities. They will often provide means for
internetworking of local networks.

Characteristics of MAN’S :

 The network size falls intermediate between LANs and WANs. A MAN
typically covers an area of between 5 and 50 km diameter. Many MANs
cover an area the size of a city, although in some cases MANs may be as
small as a group of buildings or as large as the North of Scotland.

 A MAN (like a WAN) is not generally owned by a single organisation. The
MAN, its communications links and equipment are generally owned by
either a consortium of users or by a single network provider who sells the
service to the users. This level of service provided to each user must
therefore be negotiated with the MAN operator, and some performance
guarantees are normally specified.

 A MAN often acts as a high speed network to allow sharing of regional
resources (similar to a large LAN). It is also frequently used to provide a
shared connection to other networks using a link to a WAN.

Advantages of MANs

The biggest advantage of MANs is the bandwidth (potential speed) of the
connecting links.

This means that resources (such as databases and files) shared on the
network can be accessed extremely quickly.

Some installations allow multiple users to share the same high-speed
Internet connection, thereby sharing the cost of the service and securing a
better quality of service through collective bargaining and economies of
scale.

Disadvantages of MANs

The key disadvantage of MANs is the cost of the cutting-edge technology
employed. Also, this equipment generally has to be installed for the first
time, as the copper traditionally used for the phone network is generally
considered to be too slow to be annexed for this purpose.

The cost is what inhibits the geographical reach of MANs, which is also
another drawback.

WAN:

A Wide Area Network (WAN) is a network that covers a broad area (i.e., any
network that links across metropolitan, regional, or national boundaries). The
Internet is the most popular WAN, and is used by businesses, governments,
non-profit organizations, individual consumers, artists, entertainers, and
numerous others for almost any purpose imaginable.[1]
Related terms for other types of networks are personal area
networks (PANs), local area networks (LANs), campus area networks (CANs),
or metropolitan area networks (MANs) which are usually limited to a room,
building, campus or specific metropolitan area (e.g., a city) respectively.
A computer network that spans a relatively large geographical area. Typically, a
WAN consists of two or more local-area networks (LANs).
Computers connected to a wide-area network are often connected through
public networks, such as the telephone system. They can also be connected
through leased lines or satellites. The largest WAN in existence is theInternet.

Characteristics of WAN:

Followings are the major characteristics of WAN.

1.Communication Facility: For a big company spanning over different parts
of the country the employees can save long distance phone calls and it
overcomes the time lag in overseas communications. Computer conferencing is
another use of WAN where users communicate with each other through their
computer system.

2.Remote Data Entry: Remote data entry is possible in WAN. It means sitting
at any location you can enter data, update data and query other information of
any computer attached to the WAN but located in other cities. For example,
suppose you are sitting at Madras and want to see some data of a computer
located at Delhi, you can do it through WAN.

3.Centralised Information: In modern computerised environment you will find
that big organisations go for centralised data storage. This means if the
organisation is spread over many cities, they keep their important business
data in a single place. As the data are generated at different sites, WAN permits
collection of this data from different sites and save at a single site.

Examples of WAN
1.Ethernet: Ethernet developed by Xerox Corporation is a famous example of
WAN. This network uses coaxial cables for data transmission. Special
integrated circuit chips called controllers are used to connect equipment to the
cable.

2.Aparnet: The Aparnet is another example of WAN. It was developed at
Advanced Research Projects Agency of U. S. Department. This Network
connects more than 40 universities and institutions throughout USA and
Europe.

Advantages Of WAN

-Messages can be sent very quickly to anyone else on the network. These
messages can have pictures, sounds, or data included with them (called
attachments).

-Expensive things (such as printers or phone lines to the internet) can be
shared by all the computers on the network without having to buy a different
peripheral for each computer.

-Everyone on the network can use the same data. This avoids problems where
some users may have older information than others.

-Share information/files over a larger area

Disadvantages Of WAN

-Setting up a network can be an expensive and complicated experience. The
bigger the network the more expensive it is.

-Security is a real issue when many different people have the ability to use
information from other computers. Protection against hackers and viruses adds
more complexity and expense.

-Once set up, maintaining a network is a full-time job which requires network
supervisors and technicians to be employed.

-Information may not meet local needs or interests

What is networking?

Networking is the word basically relating to computers and their connectivity. It
is very often used in the world of computers and their use in different
connections. The term networking implies the link between two or more
computers and their devices, with the soul purpose of sharing the data stored
in the computers, with each other. The networks between the computing
build and use. Networking is the word basically relating to computers and their
connectivity. It is very often used in the world of computers and their use in
different connections. The term networking implies the link between two or
more computers and their devices, with the soul purpose of sharing the data
stored in the computers, with each other. The networks between the computing
build and use.

Computer networking is then categorized into several different areas and uses,
such as the most common ones like LAN and WAN.

Computer networking is also based on different network designs. The two basic
classification categories of the network design are the client-server and peer-to-
peer. The client-server networking refers to the computer servers that are
centralized, which are mainly used in storing emails, web pages, files and
applications. The peer-to-peer network is the most commonly used and all the
computers mainly support its functions. The Client server is used extensively
in the business functions, whereas the peer-to-peer server is for home use.

Every network requires a topology to work through which the data flows and
the computers can communicate with each other. The most common types of
topologies are bus, star, ring, and mesh.

Networking also involves a special communication language used by the
computer devices. These languages are called network protocols, and most of
the computers use a range of protocols which they support. The most common
network in the Internet and home networks is the TCP/IP.

Networking can be either wired or wireless. The most common wired networks
like Ethernet cables were extensively used but now wireless networking have
emerged and the new computer networks mainly support this feature.

With the increasing use of the computers and the networking the local area
network of the LAN is one such network type which links the two computers in
a connection.

For this connection a Local area network card or the LAN card is required
which enables the connection of the computers in a network. It is a piece of
hardware which is connected inside the PC linking the computer network.

The LAN Card is of both the common types which are the OSI layer 1 and 2,
dealing with the physical as well as the data link layer respectively. It uses the
correctly entered MAC addresses for the network to work. This then allows the
computers to connect using cables or even wirelessly which then requires a
special type of LAN card called the WLAN card.

What is LAN CARD?

With the increase in the development and technology, the local area network of
the wireless type is now mostly preferred. Therefore a Wireless LAN Card is
required for this purpose. The computers with the wireless LAN Card can
transmit and receive data via radio waves using the special technology of SST
or the Spread-Spectrum technology.

The wireless LANs are available in four basic types which include the 802.11,
followed by type a, b and also g.

Any sort of LAN card you use will have some of the typical features of a
network card which includes the twisted pair, the AUI socket and also the
BNC. It is at the AUI socket that the network cable has to be connected. The
LAN cards usually are designed to support the rate transfer to be ranging from
10 to 1000 megabits per second.

HUB
A common connection point for devices in a network. Hubs are commonly used
to connect segments of a LAN. A hub contains multiple ports. When a
packet arrives at one port, it is copied to the other ports so that all segments of
the LAN can see all packets.
A passive hub serves simply as a conduit for the data, enabling it to go from
one device (or segment) to another. So-called intelligent hubs include additional
features that enables an administrator to monitor the traffic passing through
the hub and to configure each port in the hub. Intelligent hubs are also
called manageable hubs.
A third type of hub, called a switching hub, actually reads the destination
address of each packet and then forwards the packet to the correct port.

Network switch

A network switch or switching hub is a computer networking device that
connects network segments or network devices. The term commonly refers to a
multi-port network bridge that processes and routes data at the data link
layer (layer 2) of the OSI model. Switches that additionally process data at
the network layer (layer 3) and above are often referred to as layer-3
switches or multilayer switches.

The first Ethernet switch was introduced by Kalpana in 1990

Definition: A network switch is a small hardware device that joins multiple
computers together within one local area network (LAN). Technically, network
switches operate at layer two (Data Link Layer) of the OSI model.
Network switches appear nearly identical to network hubs, but a switch
generally contains more intelligence (and a slightly higher price tag) than a
hub. Unlike hubs, network switches are capable of inspecting data packets as
they are received, determining the source and destination device of each
packet, and forwarding them appropriately. By delivering messages only to the
connected device intended, a network switch conserves network bandwidth and
offers generally better performance than a hub.
As with hubs, Ethernet implementations of network switches are the most
common. Mainstream Ethernet network switches support either
10/100Mbps Fast Ethernet or Gigabit Ethernet(10/100/1000) standards.

Different models of network switches support differing numbers of connected
devices. Most consumer-grade network switches provide either four or eight
connections for Ethernet devices. Switches can be connected to each other, a
so-called daisy chaining method to add progressively

Function

A switch is a telecommunication device which receives a message from any
device connected to it and then transmits the message only to the device for

which the message was meant. This makes the switch a more intelligent device
than a hub (which receives a message and then transmits it to all the other
devices on its network). The network switch plays an integral part in most
modern Ethernet local area networks (LANs). Mid-to-large sized LANs contain a
number of linked managed switches. Small office/home office (SOHO)
applications typically use a single switch, or an all-purpose converged
device such as a residential gateway to access small
office/home broadband services such asDSL or cable Internet. In most of these
cases, the end-user device contains a router and components that interface to
the particular physical broadband technology. User devices may also include a
telephone interface for VoIP.
An Ethernet switch operates at the data link layer of the OSI model to create a
separate collision domain for each switch port. With 4 computers (e.g., A, B, C,
and D) on 4 switch ports, any pair (e.g. A and B) can transfer data back and
forth while the other pair (e.g. C and D) also do so simultaneously, and the two
conversations will not interfere with one another. In full duplex mode, these
pairs can also overlap (e.g. A transmits to B, simultaneously B to C, and so on).
In the case of a repeater hub, they would all share the bandwidth and run
in half duplex, resulting in collisions, which would then necessitate
retransmissions.
Role of switches in a network

Switches may operate at one or more layers of the OSI model, including data
link and network. A device that operates simultaneously at more than one of
these layers is known as a multilayer switch.
In switches intended for commercial use, built-in or modular interfaces make it
possible to connect different types of networks, including Ethernet, Fibre
Channel, ATM, ITU-T G.hn and 802.11. This connectivity can be at any of the
layers mentioned. While layer-2 functionality is adequate for bandwidth-
shifting within one technology, interconnecting technologies such
as Ethernet and token ring is easier at layer 3.
Devices that interconnect at layer 3 are traditionally called routers, so layer-3
switches can also be regarded as (relatively primitive) routers.
Where there is a need for a great deal of analysis of network performance and
security, switches may be connected between WAN routers as places for
analytic modules. Some vendors provide firewall,[2][3] network intrusion
detection,[4]and performance analysis modules that can plug into switch ports.
Some of these functions may be on combined modules.[5]
In other cases, the switch is used to create a mirror image of data that can go
to an external device. Since most switch port mirroring provides only one
mirrored stream, network hubs can be useful for fanning out data to several
read-only analyzers, such as intrusion detection systems and packet sniffers.

Hubs, Bridges, Switches and Routers are used to build networks. If you are
trying to design your own LAN (Local Area Network) at home, then you
probably need to know what they do and the main differences between them. I
will try to cover all that in addition to some networking details to cultivate the
article and provide better understanding of how the internet works. After all,
always remember that the internet as you know it is nothing more than a
network of networks!

Hubs are used to build a LAN by connecting different computers in a
star/hierarchal network topology, the most common type on LANs now a day. A
hub is a very simple (or dumb) device, once it gets bits of data sent from
computer A to B, it does not check the destination, instead, it forwards that
signal to all other computers (B, C, D…) within the network. B will then pick it
up while other nodes discard it. This amplifies that the traffic is shared.

There are mainly two types of hubs:

1. Passive: The signal is forwarded as it is (so it doesn’t need power supply).
2. Active: The signal is amplified, so they work as repeaters. In fact they have
been called multiport repeaters. (use power supply)

Hubs can be connected to other hubs using an uplink port to extend the
network.

OSI Model: Hubs work on the physical layer (lowest layer). That’s the reason
they can’t deal with addressing or data filtering.

Switches on the other hand are more advanced. Instead of broadcasting the
frames everywhere, a switch actually checks for the destination MAC address
and forward it to the relevant port to reach that computer only. This way,
switches reduce traffic and divide the collision domain into segments, this is
very sufficient for busy LANs and it also protects frames from being sniffed by
other computers sharing the same segment.

They build a table of which MAC address belongs to which segment. If a
destination MAC address is not in the table it forwards to all segments except

the source segment. If the destination is same as the source, frame is
discarded.

Switches have built-in hardware chips solely designed to perform switching
capabilities, therefore they are fast and come with many ports. Sometimes they
are referred to as intelligent bridges or multiport bridges.
Different speed levels are supported. They can be 10 Mb/s, 100 Mb/s, 1 Gb/s
or more.

Most common switching methods are:

1. Cut-through: Directly forward what the switch gets.
2. Store and forward: receive the full frame before retransmitting it.

OSI: Switches are on the data link layer (just above physical layer) that’s why
they deal with frames instead of bits and filter them based on MAC addresses.
Switches are known to be used for their filtering capabilities.

VLANs (Virtual LANs) and broadcast domains: Switches do not control
broadcast domains by default, however, if a VLAN is configured in a switch it
will has its own broadcast domain.

*VLAN is a logical group of network devices located on different LAN physical
segments. However they are logically treated as if they were located on a single
segment.

Bridges are used to extend networks by maintaining signals and traffic.
OSI: Bridges are on the data link layer so in principle they are capable to do
what switches do like data filtering and separating the collision domain, but
they are less advanced. They are known to be used to extend distance
capabilities of networks.

In a comparison with switches, they are slower because they use software to
perform switching. They do not control broadcast domains and usually come
with less number of ports.

Routers are used to connect different LANs or a LAN with a WAN (e.g. the
internet). Routers control both collision domains and broadcast domains. If the
packet’s destination is on a different network, a router is used to pass it the
right way, so without routers the internet could not functions.

Routers use NAT (Network Address Translation) in conjunction with IP
Masquerading to provide the internet to multiple nodes in the LAN under a
single IP address.

Now a day, routers come with hub or switch technology to connect computers
directly.

OSI: Routers work on the network layer so they can filter data based on IP
addresses. They have route tables to store network addresses and forward
packets to the right port.

Gateways are very intelligent devices or else can be a computer running the
appropriate software to connect and translate data between networks with
different protocols or architecture, so their work is much more complex than a
normal router. For instance, allowing communication between TCP/IP clients
and IPX/SPX or AppleTalk.

OSI: Gateways operate at the network layer and above, but most of them at the
application layer.

P.S. The term Gateway is used to refer to routers in some articles so beware. In
this case, the router has gateway software. And Default Gateway is used to
refer to the node (e.g. router) connecting the LAN to the outside (e.g. internet).

Repeaters are simple devices that work at the physical layer of the OSI. They
regenerate signals (active hubs does that too).

There is an important rule to obey while using repeaters/hubs to extend a local
network and is called the 5-4-3 rule or the IEEE way. The rule forces that in a
single collision domain there shouldn’t be more than 5 segments, 4 repeaters
between any two hosts in the network and only 3 of the segments can be
populated (contain user connections).
This rule ensures that a signal sent over the network will reach every part of it
within an acceptable length of time.
If the network is bigger, the collision domain can be divided into two parts or
more using a switch or a bridge.

Conclusion

What have been introduced so far are the main traditional devices used to
build networks, understanding how they work helps to understand the logic
behind networks designing, however, now that technology advance quickly, it is

possible to find new products in the market combining two or more of these
devices into one.

Examples are:

- Brouter: Works as a Bridge and as a Router.
- IP Switch or MultiLayer Switch (MLS): New switches with routing capabilities,
they forward data based on IP addresses, work at the network layer too.

Router (computing)

A Cisco ASM/2-32EM router deployed atCERN in 1987

A router is a device that forwards data packets between computer networks,
creating an overlay internetwork. A router is connected to two or more data
lines from different networks. When a data packet comes in one of the lines,
the router reads the address information in the packet to determine its
ultimate destination. Then, using information in its routing table or routing
policy, it directs the packet to the next network on its journey. Routers perform
the "traffic directing" functions on the Internet. A data packet is typically
forwarded from one router to another through the networks that constitute the
internetwork until it gets to its destination node.[1]

The most familiar type of routers are home and small office routers that simply
pass data, such as web pages and email, between the home computers and the
owner's cable or DSL modem, which connects to the Internet through an ISP.
More sophisticated routers, such as enterprise routers, connect large business
or ISP networks up to the powerful core routers that forward data at high speed
along the optical fiber lines of the Internet backbone.

Types of routers

The earliest types of EDA routers were "manual routers" -- the drafter clicked a
mouse on the endpoint of each line segment of each net. Modern PCB design
software typically provides "interactive routers" -- the drafter selects a pad and
clicks a few places to give the EDA tool an idea of where to go, and the EDA
tool tries to place wires as close to that path as possible without violating DRC.
Some more advanced interactive routers have "push and shove" features in an
interactive router; the EDA tool pushes other nets out of the way, if possible, in
order to place a new wire where the drafter wants it and still avoid violating
DRC. Modern PCB design software also typically provides "autorouters" that
route all remaining unrouted connections without human intervention.

The five main types of autorouters are:

 Maze router[1]
 Line probe router[2]
 Channel router[3]
 Area routers
 Switchbox routing

Bridge - network bridges
Definition: A bridge device filters data traffic at a network boundary. Bridges
reduce the amount of traffic on a LAN by dividing it into two segments.

Bridges operate at the data link layer (Layer 2) of the OSI model. Bridges
inspect incoming traffic and decide whether to forward or discard it. An
Ethernet bridge, for example, inspects each incoming Ethernet frame -
including the source and destination MAC addresses, and sometimes the frame
size - in making individual forwarding decisions.

Bridges serve a similar function as switches, that also operate at Layer 2.
Traditional bridges, though, support one network boundary, whereas switches
usually offer four or more hardware ports. Switches are sometimes called
"multi-port bridges" for this reason.

Modem
A modem (modulator-demodulator) is a device that modulates an analog
carrier signal to encode digital information, and also demodulates such a
carrier signal to decode the transmitted information. The goal is to produce
a signal that can be transmitted easily and decoded to reproduce the original
digital data. Modems can be used over any means of transmitting analog
signals, from light emitting diodes to radio. The most familiar example is
a voice band modem that turns the digital data of a personal computer into
modulated electrical signals in the voice frequency range of
a telephone channel. These signals can be transmitted over telephone lines and
demodulated by another modem at the receiver side to recover the digital data.
Modems are generally classified by the amount of data they can send in a
given unit of time, usually expressed in bits per second (bit/s, or bps), or bytes
per second (B/s). Modems can alternatively be classified by their symbol rate,
measured in baud. The baud unit denotes symbols per second, or the number
of times per second the modem sends a new signal. For example, the ITU V.21
standard used audio frequency-shift keying, that is to say, tones of different
frequencies, with two possible frequencies corresponding to two distinct
symbols (or one bit per symbol), to carry 300 bits per second using 300 baud.
By contrast, the original ITU V.22 standard, which was able to transmit and
receive four distinct symbols (two bits per symbol), handled 1,200 bit/s by
sending 600 symbols per second (600 baud) using phase shift keying.

What is Modem and its benefits?

Modem is actually one of the most important hardware devices used in the
computer especially for the purpose of networking of the internet. Modems are
used to modulate signals of analog types so as to be able to encode the digital
information from it. Modems are also used for demodulation of the signals so
as to decode the information transmitted.

Therefore it is for the soul purpose of transmission of the signal easily so that it
can be decoded to reproduce the digital information.

The classification of modems is done on the basis of the information that can
transferred in a given time period by them which is usually measured in bps,
or bits per second. Modems are now used by the internet users, which
preferably run faster than such as the cable and the ADSL modems.

These days the modems area great benefit to be used in the telecommunication
systems with the launch of the radio modems. These modems are the
microwave modems and can transmit even more than a hundred million bits
per second, which is a very high speed for any modem to work on. The launch
of the optical fibers has given rise to the optical modems which are now used
for the purpose of transmission of data over undersea optical fibers. These
modems have unbelievably high rate of transmission, which can reach up to
about a billion bits per second.

Although modem internet connection has been replaced over the recent years
with broadband, modems still hold a vital position is certain other aspects of
the modern world such as the space telecommunications and of course in the
telephone communications.

OSI MODEL

Virtually all networks in use today are based in some fashion on the Open
Systems Interconnection (OSI) standard. OSI was developed in 1984 by
theInternational Organization for Standardization (ISO), a global federation of
national standards organizations representing approximately 130 countries.

The core of this standard is the OSI Reference Model, a set of seven layers
that define the different stages that data must go through to travel from one
device to another over a network. In this article, you'll find out all about the
OSI standard.

Description of OSI layers

At each layer, certain things happen to the data that prepare it for the next
layer. The seven layers, which separate into two sets, are:

Application Set
Layer 7: Application - This is the layer that actually interacts with
the operating system or application whenever the user chooses to transfer files,
read messages or perform other network-related activities.
Layer 6: Presentation - Layer 6 takes the data provided by the Application
layer and converts it into a standard format that the other layers can
understand.
Layer 5: Session - Layer 5 establishes, maintains and ends communication
with the receiving device.

Transport Set
Layer 4: Transport - This layer maintains flow control of data and provides
for error checking and recovery of data between the devices. Flow control
means that the Transport layer looks to see if data is coming from more than
one application and integrates each application's data into a single stream for
the physical network.
Layer 3: Network - The way that the data will be sent to the recipient device is
determined in this layer. Logical protocols, routing and addressing are handled
here.
Layer 2: Data - In this layer, the appropriate physical protocol is assigned to
the data. Also, the type of network and the packet sequencing is defined.

Layer 1: Physical - This is the level of the actual hardware. It defines the
physical characteristics of the network such as connections, voltage levels and
timing.

The OSI Reference Model is really just a guideline. Actual protocol
stacks often combine one or more of the OSI layers into a single layer.

According to recommendation X.200, there are seven layers, labeled 1 to 7,
with layer 1 at the bottom. Each layer is generically known as an N layer. An
"N+1 entity" (at layer N+1) requests services from an "N entity" (at layer N).
At each level, two entities (N-entity peers) interact by means of the N protocol
by transmitting protocol data units (PDU).
A Service Data Unit (SDU) is a specific unit of data that has been passed down
from an OSI layer to a lower layer, and which the lower layer has not yet
encapsulated into a protocol data unit (PDU). An SDU is a set of data that is
sent by a user of the services of a given layer, and is transmitted semantically
unchanged to a peer service user.
The PDU at a layer N is the SDU of layer N-1. In effect the SDU is the 'payload'
of a given PDU. That is, the process of changing an SDU to a PDU, consists of
an encapsulation process, performed by the lower layer. All the data contained
in the SDU becomes encapsulated within the PDU. The layer N-1 adds headers
or footers, or both, to the SDU, transforming it into a PDU of layer N. The
added headers or footers are part of the process used to make it possible to get
data from a source to a destination.
OSI Model

Data unit Layer Function

7. Application Network process to application

Data representation, encryption
and decryption, convert machine
6. Presentation
dependent data to machine
Data
Host independent data
layers
Interhost communication,
5. Session managing sessions between
applications

Segments 4. Transport End-to-end connections, reliability

and flow control

Path determination and logical
Packet/Datagram 3. Network
addressing

Media
Frame 2. Data link Physical addressing
layers

Media, signal and binary
Bit 1. Physical
transmission

Some orthogonal aspects, such as management and security, involve every
layer.
Security services are not related to a specific layer: they can be related by a
number of layers, as defined by ITU-TX.800 Recommendation.[3]
These services are aimed to improve theCIA triad (confidentiality, integrity,
andavailability) of transmitted data. Actually the availability of communication
service is determined by network design and/ornetwork
management protocols. Appropriate choices for these are needed to protect
against denial of service.
Layer 1: physical layer
The physical layer defines electrical and physical specifications for devices. In
particular, it defines the relationship between a device and a transmission
medium, such as a copper or fiber optical cable. This includes the layout
of pins, voltages, line impedance, cable specifications, signal
timing, hubs, repeaters, network adapters, host bus adapters (HBA used
in storage area networks) and more.
The major functions and services performed by the physical layer are:

 Establishment and termination of a connection to
a communications medium.
 Participation in the process whereby the communication resources are
effectively shared among multiple users. For example, contention resolution
and flow control.
 Modulation or conversion between the representation of digital data in user
equipment and the corresponding signals transmitted over a
communications channel. These are signals operating over the physical
cabling (such as copper and optical fiber) or over a radio link.
Parallel SCSI buses operate in this layer, although it must be remembered that
the logical SCSI protocol is a transport layer protocol that runs over this bus.
Various physical-layer Ethernet standards are also in this layer; Ethernet
incorporates both this layer and the data link layer. The same applies to other

local-area networks, such as token ring, FDDI, ITU-T G.hn and IEEE 802.11,
as well as personal area networks such as Bluetooth and IEEE 802.15.4.
Layer 2: data link layer
The data link layer provides the functional and procedural means to transfer
data between network entities and to detect and possibly correct errors that
may occur in the physical layer. Originally, this layer was intended for point-to-
point and point-to-multipoint media, characteristic of wide area media in the
telephone system. Local area network architecture, which included broadcast-
capable multi-access media, was developed independently of the ISO work
in IEEE Project 802. IEEE work assumed sublayer-ing and management
functions not required for WAN use. In modern practice, only error detection,
not flow control using sliding window, is present in data link protocols such
as Point-to-Point Protocol(PPP), and, on local area networks, the IEEE
802.2 LLC layer is not used for most protocols on the Ethernet, and on other
local area networks, its flow control and acknowledgment mechanisms are
rarely used. Sliding window flow control and acknowledgment is used at the
transport layer by protocols such as TCP, but is still used in niches
where X.25 offers performance advantages.
The ITU-T G.hn standard, which provides high-speed local area networking
over existing wires (power lines, phone lines and coaxial cables), includes a
complete data link layerwhich provides both error correction and flow control
by means of a selective repeat Sliding Window Protocol.
Both WAN and LAN service arrange bits, from the physical layer, into logical
sequences called frames. Not all physical layer bits necessarily go into frames,
as some of these bits are purely intended for physical layer functions. For
example, every fifth bit of the FDDI bit stream is not used by the layer.

Layer 3: network layer
The network layer provides the functional and procedural means of transferring
variable length data sequences from a source host on one network to a
destination host on a different network (in contrast to the data link layer which
connects hosts within the same network), while maintaining the quality of
service requested by the transport layer. The network layer performs
network routing functions, and might also perform fragmentation and
reassembly, and report delivery errors. Routers operate at this layer, sending
data throughout the extended network and making the Internet possible. This
is a logical addressing scheme – values are chosen by the network engineer.
The addressing scheme is not hierarchical.
The network layer may be divided into three sublayers:

1. Subnetwork access – that considers protocols that deal with the interface
to networks, such as X.25;
2. Subnetwork-dependent convergence – when it is necessary to bring the
level of a transit network up to the level of networks on either side
3. Subnetwork-independent convergence – handles transfer across multiple
networks.
An example of this latter case is CLNP, or IPv6 ISO 8473. It manages
the connectionless transfer of data one hop at a time, from end system
to ingress router, router to router, and from egress router to destination end
system. It is not responsible for reliable delivery to a next hop, but only for the
detection of erroneous packets so they may be discarded. In this scheme, IPv4
and IPv6 would have to be classed with X.25 as subnet access protocols
because they carry interface addresses rather than node addresses.
A number of layer-management protocols, a function defined in the
Management Annex, ISO 7498/4, belong to the network layer. These include
routing protocols, multicast group management, network-layer information and
error, and network-layer address assignment. It is the function of the payload
that makes these belong to the network layer, not the protocol that carries
them.
Layer 4: transport layer
The transport layer provides transparent transfer of data between end users,
providing reliable data transfer services to the upper layers. The transport layer
controls the reliability of a given link through flow control,
segmentation/desegmentation, and error control. Some protocols are state-
and connection-oriented. This means that the transport layer can keep track of
the segments and retransmit those thafail. The transport layer also provides
the acknowledgement of the successful data transmission and sends the next
data if no errors occurred.
OSI defines five classes of connection-mode transport protocols ranging from
class 0 (which is also known as TP0 and provides the least features) to class 4
(TP4, designed for less reliable networks, similar to the Internet). Class 0
contains no error recovery, and was designed for use on network layers that
provide error-free connections. Class 4 is closest to TCP, although TCP
contains functions, such as the graceful close, which OSI assigns to the
session layer. Also, all OSI TP connection-mode protocol classes provide
expedited data and preservation of record boundaries. Detailed characteristics
of TP0-4 classes are shown in the following table

Feature Name TP0 TP1 TP2 TP3 TP4

Connection oriented network Yes Yes Yes Yes Yes

Connectionless network No No No No Yes

Concatenation and separation No Yes Yes Yes Yes

Segmentation and reassembly Yes Yes Yes Yes Yes

Error Recovery No Yes Yes Yes Yes

Reinitiate connection (if an excessive number
No Yes No Yes No
of PDUs are unacknowledged)

Multiplexing and demultiplexing over a single virtual
No No Yes Yes Yes
circuit

Explicit flow control No No Yes Yes Yes

Retransmission on timeout No No No No Yes

Reliable Transport Service No Yes No Yes Yes

An easy way to visualize the transport layer is to compare it with a Post Office,
which deals with the dispatch and classification of mail and parcels sent. Do
remember, however, that a post office manages the outer envelope of mail.
Higher layers may have the equivalent of double envelopes, such as
cryptographic presentation services that can be read by the addressee only.
Roughly speaking, tunneling protocols operate at the transport layer, such as
carrying non-IP protocols such as IBM's SNA or Novell's IPX over an IP
network, or end-to-end encryption with IPsec. While Generic Routing
Encapsulation (GRE) might seem to be a network-layer protocol, if the

encapsulation of the payload takes place only at endpoint, GRE becomes closer
to a transport protocol that uses IP headers but contains complete frames or
packets to deliver to an endpoint. L2TP carries PPP frames inside transport
packet.
Although not developed under the OSI Reference Model and not strictly
conforming to the OSI definition of the transport layer, the Transmission
Control Protocol (TCP) and the User Datagram Protocol (UDP) of the Internet
Protocol Suite are commonly categorized as layer-4 protocols within OSI.
Layer 5: session layer
The session layer controls the dialogues (connections) between computers. It
establishes, manages and terminates the connections between the local and
remote application. It provides for full-duplex, half-duplex,
or simplex operation, and establishes checkpointing, adjournment,
termination, and restart procedures. The OSI model made this layer
responsible for graceful close of sessions, which is a property of
the Transmission Control Protocol, and also for session checkpointing and
recovery, which is not usually used in the Internet Protocol Suite. The session
layer is commonly implemented explicitly in application environments that
use remote procedure calls. On this level, Inter-Processcommunication happen
(SIGHUP, SIGKILL, End Process, etc.).
Layer 6: presentation layer
The presentation layer establishes context between application-layer entities, in
which the higher-layer entities may use different syntax and semantics if the
presentation service provides a mapping between them. If a mapping is
available, presentation service data units are encapsulated into session
protocol data units, and passed down the stack.
This layer provides independence from data representation (e.g., encryption) by
translating between application and network formats. The presentation layer
transforms data into the form that the application accepts. This layer formats
and encrypts data to be sent across a network. It is sometimes called the
syntax layer.[5]
The original presentation structure used the basic encoding rules of Abstract
Syntax Notation One (ASN.1), with capabilities such as converting an EBCDIC-
coded text file to anASCII-coded file, or serialization of objects and other data
structures from a
Layer 7: application layer
The application layer is the OSI layer closest to the end user, which means that
both the OSI application layer and the user interact directly with the software
application. This layer interacts with software applications that implement a
communicating component. Such application programs fall outside the scope
of the OSI model. Application-layer functions typically include identifying

communication partners, determining resource availability, and synchronizing
communication. When identifying communication partners, the application
layer determines the identity and availability of communication partners for an
application with data to transmit. When determining resource availability, the
application layer must decide whether sufficient network or the requested
communication exists. In synchronizing communication, all communication
between applications requires cooperation that is managed by the application
layer.

BIBLIOGRAPHY

WWW.GOOGLE.COM

WWW.WIKIPEDIA.COM

http://computer.howstuffworks.com

Networking hard copy (1)

Empfohlen

Empfohlen

Weitere ähnliche Inhalte

Was ist angesagt?

Was ist angesagt? (19)

Ähnlich wie Networking hard copy (1)

Ähnlich wie Networking hard copy (1) (20)

Networking hard copy (1)