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INTRODUCTION TO COMPUTER NETWORKS




INTRODUCTION TO COMPUTER NETWORKS

     BY
FELIPE MARTÍNEZ MONDÉJAR
COMPUTER SCIENCE
4t E.S.O

 

INDEX

1 TYPE OF NETWORKS

1.1 LANS. Local Area Network

1.2 Wide Area Lans    (WAN)

1.3 Metropolitan Area Networks

1.4 Home Area Network (HAN)

2. NETWORK DEVICES

2.1 CLASSIFICATION BY LEVEL

2.2. NETWORK INTERFACES

3. TRANSMISSION MEDIA

3.1 TWISTED PAIR CABLE

3.2 OPTICAL FIBER

4. SWITCHES

5. IP ADRESSES

6. ROUTERS

7. DHCP SERVICE

8. DNS SERVICE

9. PUBLIC AND PRIVATE ADDRESSES

10. WIRELESS NETWORKS

 

 

1. TYPES OF COMPUTER NETWORKS

 

1.1 Local Area Networks

The most  spread networks in organizations are LAN (Local area networks). A Local Area Network is limited to a small area, usually a building or a campus of maximum 200m.

 

Nowadays, most L.A.N.'s are  Ethernet networks. These networks are characterized by using twisted pair and fiber optic cable as transmission media. Networks using Ethernet divide a stream of data into shorter pieces called frames. Each frame contains source and destination addresses and error-checking data so that damaged frames can be detected, discarded and re-transmitted. The Ethernet standards encompass several wiring and signaling variants. The original Ethernet used a coaxial cable similar to those of TV as a shared medium. Later the coaxial cables were replaced with twisted pair and fiber optic links in conjunction with hubs or switches.

 

Nowadays there are several forms of Ethernet networks. The most widespread are  Fastethenet which allows a speed of  100Mb / s. Also Gigabit Ethernet networks are becoming common (up to 1000Mb/s)

 

 

Some L.A.N. examples:

 

 


 

 

 

 

1.2 Wide Area Networks

 

A wide area network is a set of local area networks that extend covering a large geographical area and which are interconnected by an infrastructure of high speed links. Usually they use optical fiber or Serial high performance wired connections in the backbone, allowing data transfers between the different LAN's

 

 

 

1.3 Metropolitan area network (MAN)

A metropolitan area network (MAN) is a network that interconnects users with computer resources in a geographic area or region larger than that covered by even a large local area network (LAN) but smaller than the area covered by a wide area network (WAN).

1.4 Home Area Network (HAN)

A home network or home area network (HAN) is a type of local area network with the purpose to facilitate communication among digital devices in a home. The importance of this networks are steadily rising, due to the spread of digital household devices able to connect to Internet.  


ACTIVITY 1 : Working in groups discuss different examples of networks you know. Write a table with the networks your group has come up with, including the name of the organization, the type of organization and the number of computers you think it is made up. Classify them according their size in  LAN, WAN, MAN and HAN. They must be real networks you, your family, friends or relatives are in in contact or related in some way.


  2. NETWORK DEVICES


2.1 DEVICES CLASSIFICATION

Within a network the devices  can be classified by function  as:

 

CLASSIFICATION

TYPE OF DEVICE

EXAMPLES

LEVEL 1

End devices

Computers,Printers,Tablets,Fax, SMART TV'S, Game Consoles, mobile phone's

LEVEL 2

Devices used to connect devices inside the LAN

Hubs,switches

LEVEL3

Used to connect devices from DIFFERENT LAN's

Routers

Later you will find out why this classification is so important in order to achieve a feasible network simulation.

 

 

2.2 NETWORK INTERFACES

 

A Network card is a device that connects a computer to a computer network, usually a LAN. The Network card is also called a network adapter or NIC (Network Interface Card). There are different types of adapters depending on the type of wire used, (fiber optic, twisted pair), or the architecture used in the network (Ethernet, Fast Ethernet, Gigabit Ethernet, serial interfaces), but currently the most common is the FastEthernet using an RJ-45 connector and twisted pair cable.

Fast Ethernet Network Interface

 

Serial Network Interface



 

 

 

 

The MAC address ("media access control") is an identifier of 48 bits (6 bytes, each represented by two hexadecimal digits) that identifies uniquely a network card. It is also known as physical address. MAC addresses are globally unique, since they are written directly, in binary form, in the network card upon being made. There aren't two NICs in the world with the same MAC address

example of MAC address

 

 

 
FA:21:11:62:8E:C4

 

 

3. TRANSMISSION MEDIA

3.1 Twisted pair cable

This cable is made up of a variable number of copper wire pairs that are entangled each other to reduce the electromagnetic noise (interference).

 

This type of cable is used massively in most local area networks. The Ethernet lan's uses a four twisted pair cable ended in an RJ45 connector like the one shown int the bottom figure.

In Shielded Twisted pair cables (STP) there is a sheet of foil covering the twisted wire pairs to enhance their protection from external interference. This allows increased performance at the cost of a higher price. For this reason this cables are only used in special purposes. The Unshielded variant (known as UTP cable) is the conventional cable found in a typical LAN.

 

 

 

 

Twisted pair cable

RJ45 plug

Shielded Twisted Pair cable (STP)


 


 

Twisted pair cable can also be classified  in categories (CAT-1, CAT-2.. and so on) by its quality level. The most widespread cable in today's networks are STP and UTP Category 5 (CAT 5) cable  for both Fastethernet (100mb/s) and Gigabit networks (1000mb/s). Category 6 and 7 ( CAT 6 and CAT 7) cables are improved versions of higher quality and price use for special purposes.

 

There are two types of twisted pair connections. The difference is very important in order to carry out correctly the activities that we will perform in the following sessions:

 
 

The twisted pair:

 Allows high transmission rates (up to 1 gigabit / s, ie 1024mb / s)
  1. it is cheap

  2. It can only be used over short distances (always less than 100m). This limits their use to Local Area Networks

ACTIVITY 2 In this activity we will connect two Pcs using a twisted pair cable

 

 

 

 

 

 

 

 

 

 

 


ACTIVITY 3. Discuss with your team pals what's the cause of the problem set forth in the previous paragraph. Consider the following hypothesis and discuss about them.  You must come up with a list of points supporting or discarding  them.
a) It's impossible to connect two computers by  cable. It's necessary a wireless connection
b) We are using direct cable. we should use a crossover cable due to the devices level
c) We are using crossover cable. We should use a straight cable due to the devices level.
d) We should use a optical fiber cable due to the red dots



CONNECTION WITH CROSSOVER CABLE

 

 

 

 

 
 

Now, we will explore the network card of the PC's in order to find out their physical address (MAC address)

 

 

 

Save the file and upload it to moodle


3.3 Optical fiber

 

With optical fiber cable connectors

Optical fiber cable

 

 

 

 

ACTIVITY 4. Connecting two PCS through a optical fiber  cable.

In this activity we will do the same we did in the previous session, but connecting the PCs with another type of cable: optical fiber. The computers now have an Ethernet network card with an RJ45 socket, so the first thing we have to do is to change the network card.

 
 
 
 




Drag the card out of the PC by clicking on it and dragging the rectangle in the lower right corner out of the window

 

 

 
 

 

 

 

 

Save your activity and upload it to Moodle

 

4. SWITCHES



Two PC's connected by a cable are not a computer network. What if we want to connect 10 computers together? Do we need to install 10 Network cards in any PC in order to get them interconnected?? That's not obviously a good solution.

 
To connect multiple computers to the same network we must use a type of device called a SWITCH. A switch is a device with several integrated Ethernet ports, that allows the connection of multiple devices in the same LAN. The switch provides other services as signal regeneration, some security issues and specially, it offers tools for centralized management of the network.

Activity 5. Setting up a 4 computers network

 

 

 
 

 

Save the file and upload it to Moodle

 

 

 

ACTIVITY 6. SETTING UP A RING NETWORK

A ring network is a network in which all computers are connected together forming a ring

 

 

The main drawback of ring networks is that in order to close the ring, you need two network cards per computer. In Packet Tracer a generic PC  has only a network card. Therefore instead of using generic PCs desktop we will use a device called Generic Server

 
In the following picture, you can see the Generic Server  highlighted with a yellow circle. If you click on the server the configuration window will open. Select the physical tab. There is a free slot for a second network card (marked in the picture with a red circle).




 

5. IP ADDRESS



Building  a computer network is not  as simple as to wire several computers. When a data packet is transmitted, it must be clear for all the participants who is the sender host, and who is the addressed host. In the telephone network this problem is solved by the use of telephone numbers:when  a call is made ​​the recipient's phone number must be dialed.  In computers the solutions follows a similar but more complex pattern called routing.
 

We  know yet an addressing mechanism in networks: the physical addressing or Mac address in the network cards. However this type of addressing, is not suitable for large networks for different reasons. One of the most important is that for users would it's too difficult to dial a hexadecimal number of 8 digits as 0030A3AA570C

 

Internally, computers directly wired use physical addressing to communicate among them. However, both for small networks as for huge ones as the Internet, is used a routing mechanism called IP address (IP stands for Internet Protocol). In brief, this is how it works:

 
 

The IP address is made up of 4 digits ranging from 0 to 255 and separated by a dot. Examples of proper IP addresses would be:

192.168.100.2

221.200.0.11

11.23.2.112



E    Examples of incorrect IP addresses would be:

258 .33.2.3- There's a digit over 255

0 .3.2.3 - The first digit can not be 0

255.255 .243.12- address 255 is reserved

232.223.44.33 .22- are five digits instead of 4



The IP address is divided in two parts

 

 

In the image above there are two networks. The network on the left has a 192.168.18.0  network address. All computers in each of the networks share the same network address and differ by having a different  host number. Therefore in the left network the computers are identified by the following IP addresses

192.168.18 . 99

192.168.18 .11

192.168.18 .22

192.168.18 .33

 

The network identifier of the right network is 192.168.5.0. and the host numbers are:

 

 192.168.5.11

 192.168.5.22

 192.168.5.33



Pay attention to the following statement because it is very important.

 

In an IP network, although two computers are connected to the same cable, they can only communicate with each other, if they have the same network address

 

A mask is used in order to distinguish which part of the IP address is the network address and the host address. The mask is a 4 digits among 0 and 255 and separated by points. Though in real networks there are many different masks, in this unit we will consider only three possible masks


Mask

Meaning

255.0.0.0

The first digit is the network address, the rest are station addresses

255.255.0.0

The first and second digits are the network address, the rest are station addresses

255.255.255.0

The first, second and third digits are the network address, the rest are station addresses



E.g.

IP Address

Mask

Network address

Host address

221.23.22.33

255.255.0.0

221.23.0.0

22.33

180.12.22.1

255.255.255.0

180.12.22.0

1

21.23.222.1

255.0.0.0

21.0.0.0

23.222.1

123.12.3.1

255.255.0.0

123.12.0.0

.3.1


89.234.33.22


255.255.255.0


89.234.33.0


22

 

Activity 7 Designing Ip addresses for a 8 hosts network.

 

 

 

Now all PCs seem to be connected. However they still can not send or receive anything because they have no IP addresses neither any network masks

 





Just fill in the IP Configuration section with the following data

 

 

 



 

Configure the other computers with the following IP addresses and masks, following the explained procedure


Computer

IP Address

Masks

PC1

192.168.30.1

255.255.255.0

PC2

192.168.30.2

255.255.255.0

PC3

172.16.20.3

255.255.0.0

PC4

172.16.20.4

255.255.0.0

PC5

192.168.30.5

255.255.255.0

PC6

192.168.30.6

255.255.255.0

PC7

172.16.20.7

255.255.0.0

PC8

172.16.20.8

255.255.0.0


Notice that what we have done is to set up two different networks.

 

 

 

Remember the rule: although they are connected, only those computers who have the same network address can exchange data packets. We are going to check out  whether this rule is true

 

 



 

 




As you can see when PC1 pings to a computer that is located in a different network address there is no answer. All the frames are lost

Save the file  and upload it to Moodle

6. ROUTERS



So far, we have learned that it is not possible to connect two computers that are not configured with an IP address belonging to the same network address. However this does not match at all with our everyday experience. When we connect to the internet, we are able to connect with millions of computers each of which is in a completely different network address. This is possible thanks to another kind of devices: the routers.

A router is a device that allows connection between networks with different network addresses. An example

In the network shown in the image there are two computer networks, both with the mask 255.255.255.0. PCs from the top left network belong to the 192.168.200.0 network address, and the bottom right computers are on the  192.168.1.0 address network. Therefore they are not able to  communicate each other.

 

The router is the round sampled device in the middle of the network. Notice it is linked with both networks. A router is a third level device, which two or more network interfaces in order to be connected to two or more networks. The function of the router is to allow communication between the computers on different networks

 

When H8 wants to send a data packet to H3 wich is a computer out of its own network, it sends it to the Router. The router looks into  the destination address, and 'routes' the packet to the correct output (that is, it sends the packet for the network interface connected to the network where  H3 is located)

 

Therefore the computers in a network, in order to be able to communicate with other networks, must be configured with the IP address and network mask of the router interface connected to them. This interface is called a GATEWAY.

ACTIVITY 8. Using CISCO Packet Tracer design the network shown in the figure above.

 

 
 

 

 

We must test if it is working. If all goes right, we will be able to ping from H3 to H8 and receive a "pong".

 

Save the activity as activity 6 and upload it to Moodle


ACTIVITY 9 Make the Quiz 1 in Moodle.  It's a 10 questions test with multiple choice answers. You can use both this text and any information you find on the Internet so solve it. But keep in mind that you will be allowed only one attempt.

1. The optical fiber  cable is suitable for data transmission
Select one:

a. High speed and over long distance
b. Slow speed, high distance connections
c. High speed but a short distance connections
d. High speed and wireless connections


2.Twisted pair cable consists of a variable number of pairs of wires 
Select one:

a. That  are entangled each other to reduce the electromagnetic noise
b. That are welded together to reduce the burden and electromagnetic noise
c. That are entangled  so that the cable has more strength and support greater weights
d. That stick together to be more resilient


3.To directly connect two devices of the same type (two routers, or two switches, for example) it is necessary to unite them with:
Select one:

a. It can only be done through a twisted pair parallel cable
b. It can only be done with a twisted pair crossed
c. It can only be done with a optical fiber cable
d. It uses direct twisted pair cable


4.The physical address or MAC address is:
Select one:

a. An address of each computer that is recorded in factory
b. The number of computer chassis
c. A number of network card that is recorded in factory
d. The logical network address
 
 

5. Devices A and C
Select one:

a. They are routers, and connect computers with the same network address
b. They are switches and connect computers, which can only communicate with each other if they have the same network address
c. They are switches and connect computers, which can only communicate with each other if they have the same host address
d. They are routers and enable communication between computers with different network address

6. To form a network, computers must...
Select one:

a. They must be connected or must have an IP address
b. They must be physically connected, and have an IP address with the same network address
c. They must be physically connected by the same cable
d. They must be connected and have the same IP address

 

7. 195.11.195.1 is a valid IP address
Select the correct:

  1. true
  2. false

8. If  PC2 pings PC5, will it get an answer?

  1. true
  2. false

9. 255.11.114.1 is a valid IP address
Select one:

  1. true
  2. false

10. The discontinnued line between the two switches indicates that the connection is intermittent

  1. true
  2. false

 

7. DHCP

 

So far, we are setting the IP addresses manually in every computer. In a network with many computers scattered throughout a large geographical area this brings about lots of work and serious problems. For instance: two different computers might  get accidentally the same IP address, that's a serious problem. Or someone will have to change the IP address anytime a computer is changed of LAN. Thats  a lot of work. There is a protocol called DHCP that provides a better solution. In DHCP, whenever a network computer boots up, it looks for a DHCP server on the network, and ask it an IP configuration. DHCP servers assigns correct IP addresses avoiding repeated configurations in the network

ACTIVITY 10. DHCP


 

 

 

 

 
 

 

 
 

 

 
 
 
 
 
 

 

 
 
 

 

 

 

Let's try if our DHCP server is working.

 

 



Upload to Moodle the  Activity 10

 

8. Domain Name System (DNS)

 

 
 

Domain Name System or DNS is a system for naming computers connected to the Internet or a private network . For most people it's difficult to memorize numbers as an IP addresses. Instead humans are good at memorizing words, so it is much easier for a human to remember something like www.amazon.com than its equivalent IP address. This is the reason why the DNS system was developed: to translate (resolve) names intelligible to humans to IP addresses with the purpose of locating and routing equipment worldwide on the Internet.



 

 

DNS names are based on a hierarchy. All Internet names must have a primary domain name as com, org, net, UN, es, fr, it etc. These names are useful classifying Internet servers by its main function. Commercial companies take the .com domain. The nonprofit organizations often use .org. Another possibility is to identify the country where their server is located. In the case of Spanish organization and companies the primary domain is .es

 
 

Left to  the primary domain, and separated by a dot, comes the domain name of the organization. This is the word used to identify that company on the Internet. Examples are amazon.com, ebay.com, elpais.es, hotmail.com.

 
 

Some organizations add a subdomain. For example the Valencian Goverment uses the domain gva.es. But the Education Department,  uses the domain edu.gva.es

 
 

The last component of a DNS name is the name of the server within the domain. For example, the Antares computer, in the Education Department would use the following dns name:

 

antares.edu.gva.es

 
 

By the way, when accessing any website with a name that begins with www, for instance: www.elpais.es , we are connecting to a computer named www, which belongs to the specified domain, in this case elpais.es . Organizations always puts his web servers the name www, to make predictable for user the name of their web site.

DNS servers

On Internet someone has to do the translation (resolution) of a name into an IP address and vice versa. The Internet consists of billions of computers, so it is impossible to put all them in a unique data base shared by all DNS servers. This Database would be too huge and it would be impossible to  deal with all the constant updates. DNS is based on a distributed database in which there are multiple DNS servers each responsible for its own section of  Internet and depending on a higher authority over them. For this reason has been  established the  following hierarchy of DNS servers

 
 

When a user puts in your browser the URL www.wikipedia.com  the resolution of this name goes as follows:

 

 

 

Finally the local DNS server answers the question asked for your browser, handing him in the IP address of www.wikipedia.org

ACTIVITY 11. Your group must search on Internet information about the root servers on Internet, and then answer the following questions:

 
Upload your answers to moodle

 

ACTIVITY 12. INSTALLING A DNS SERVER

 

 
 

 

  • PC0

  • 172.16.10.10/255.255.0.0

  • PC1

  • 172.16.10.11/255.255.0.0

  • PC2

  • 172.16.10.12/255.255.0.0

  • PC3

  • 172.16.10.13/255.255.0.0



 

 

 

 

 

 

 

 

 


 

 


 

 

 


 

 

 

 

 

 

 

 

 



 


 



 

 

UPLOAD TO MOODLE THE ACTIVITY


9. PUBLIC AND PRIVATE IP ADDRESSES

 

When the IP protocol was developed, it was thought that 32 bits (four numbers from 0 to254) would be enough to assign an IP address to  every computer in the world, because there were 4.294.967.296  of them.

 

This number of IP addresses can seem enormous. But actually the world has nearly run out of them.

 


 

ACTIVITY 13:

 

Work out how many IP addresses do you need in your home. You must add up an IP address for every mobile phone, every tablet, every PC, every e-Book (most allow Internet access), each Smart TV, game console, media players, network disks, NAS servers and any other wireless device in your home ( yours or belonging to your family members).  Nowadays even Power and water supply counters are connected to Internet.

 

Fill the following table:

 

DEVICE  STUDENT 1 STUDENT 2 STUDENT 3 TOTAL
Family members        
Computers        
Mobile phones        
Tablets        
SMART TV        
E-Books        
Game consoles        
iPODs -Mp3's        
DIGITAL  COUNTERS (Power and water suply)        
Other devices        
TOTAL        

Estimate the number of IP addresses you, and your family members need in other places as  in their job, their bussiness or your school.

 

  student 1 Student 2 Student 3 TOTAL
At work/business        
At school/University        
TOTAL        


Taking into account the tables above calculate the average number of Ip addresses per person  in the developped world.

Estimate how many Ip addresses are needed in the world. Consider  there are about 2 billion people in the developed wold, 7 billion people in global. And estimate  the average for  people in the undeveloped countries as 1/4 the figure you have calculated for the developed world.

 

what do you think, the world is running out of IP addresses or there are still plenty of them??

 

Upload your answers to moodle

 

 

There are several solutions to this problem: the main solution is  the IPV6 protocol. IPv6 addresses are made of 128 bits (32 hexadecimal digits), which results in 2138 IPv6 addresses. This means a staggering 6.7 trillion IPV6 addresses per square millimeter on the planet. It is foreseeable that there wont be any  scarcity of IPv6 address over a long period of time.

 

But until IPv6 is fully introduced, the only way to save IP addresses is the world is the  private and public system of IP's directions.

 
 
 

 

 

Using this trick we save many millions of public IP addresses. Before this system, an organization with 1000 computers needed 1000 public IP's. Now it only might need one Ip public address for its router. The other 999 computers would use private IP and when they would need to access the Internet, they will ask its  router to do it for them. Private IP addresses can be reused again and again in computers belonging to differents LANs reducing drastically the number of public IP's the world needs.

 

At home for example, even if you have multiple network enabled devices, only the router has a public IP address provided by your ISP. Other devices on your local network are private IP addresses assigned by your router.

 

The following table shows the private IP addresses ranges

 




ACTIVITY 14. public IP addresses.

 

In order to do this activity you need to log in our Moodle  and download the file simulación_internet.pka. In this file a cloud of routers simulates how the Internet works.

 

 

 



 

 
 

 

UPLOAD TO MOODLE  THIS ACTIVIY

 

ACTIVITY 15 local network for a school.


In this activity we are going to build a local network for a school where there will be two classrooms equipped with 10 computers.

 

 

 

 
 

Save the file and upload it  to moodle.

 

ACTIVITY 16. Local network with DHCP server

In the previous activity we have built a small local network with static IP addresses, that is, the network administrator (you)  had to set up each computer IP individually.  However, in a Linksys wireless router, we can enable the DHCP service so that the router could assign IP addresses dynamically.

Open the file Simulación_internet.pka  and save it as activity16.

Follow the next steps:

 

 

 

 

 

 

10. WIRELESS COMMUNICATION (WIFI)

 



Wireless communication (WIFI)

WIFI is a mechanism for connecting electronic devices wireless. In a wireless communication the medium  used is shared (the cable is the air!!!), and this has many negative implications. However the advantages make it afford to get over the problems: the stations can be mobile and no cable is needed. That is the reason why WIFI networks are thriving

 

 

 
Wi-Fi:  is a trademark of the Wi-Fi Alliance, this organization tests and certificates that the equipment meets the standards related to wireless local area networks.

802.11 : The 802.11 standards establish a series of requirements that must meet the network devices to be considered compatible.

The standards IEEE 802.11b , IEEE 802.11g and IEEE 802.11n operates in the band of 2.4 GHz with a speed of up to 11 Mbit / s , 54 Mbit / s and 300 Mbit / s, respectively.

 

ACTIVITY 17. WIRELESS NETWORK

In  this activity we are going  to recreate a home network for a family of three, Juan, Maria, and his son Juanito

 

 




 

 

 








=



 

Router as Fast Ethernet and Wireless SWITCH

 

 

 

 


 

The router as DHCP server


This type of router is designed to provide a home user everything he could need to set up a home network.  The router integrates a DHCP server to provide private IP addresses to each of the connected devices. Just connect the computers to the router by wify or cable and  it will get a full Ip configuration.

 

 

Let's check connectivity by pinging between the PC of John, Mary, Iphone and Xbox.

 

 

 

 

Connecting the Linksys to the router 110.

 
 
 

 

 


 




 







=






But in our simulation the router 160 is not a DHCP server and  the IP address of the router Ethernet interface have to be set up manually.

 

 



 

Save the file and upload it to Moodle.

 

 

Short url:   http://multidict.net/cs/2946