Identify network hardware and protocols Identify network hardware and protocols



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8  Network address translation


  • You will have read about NAT when exploring IP packet forwarding, but it is covered in more detail here.

  • If you examine the diagram below, you will see that the home LAN is using IP addresses in IP network 192.168.0.0/24, which will be the case for all the ISP’s customers who are using the same type of home router. This will cause problems, as the source and destination addresses have to be unique in every communication unicast (sending packets between two devices).

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Figure 21

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  • Why do home routers use the same address on the home LAN if it will cause problems within the Internet? Remember, IPv4 cannot provide sufficient IP addresses for all the devices that want to use IP, so certain ranges of IP address, including 192.168.0.0/16, are set aside as private addresses.

  • Private IP addresses can be used by anyone, at any time, within a private network. This addresses the issue of not having sufficient IP addresses, but creates a problem in that the IP address cannot be used to communicate over the Internet as it is not unique. However, you will have a unique, public IP address assigned to your home router on the interface that connects to the ISP, which in the diagram above is 82.10.250.19.

  • The idea of NAT is to convert the source address from all outgoing LAN packets into the unique public address assigned to the home router, and vice versa for incoming packets from the Internet.

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IP addresses in packets



  • The image below shows a packet transmitted from the home PC towards the web server, which needs to be routed towards the Internet by the home router:

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Figure 22

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  • Note that in the diagram above the source address is 192.168.0.101, which is private.

  • The packet will be received by the home router, which will perform NAT, converting the source address to its own WAN interface IP address (82.10.250.19):

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Figure 23

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  • The packet can now be forwarded through the Internet to the destination web server, which will return the requested webpage in a series of packets:

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Figure 24

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  • The IP packet is now addressed with the web server acting as the source, and the public address of the home router WAN interface as the destination. The home router will accept the incoming packet, and translate the destination address back to the private IP address of the home PC:

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Figure 25

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  • The use of NAT and private IP addresses has extended the life of IPv4 well beyond what would have been possible with the original range of addresses it provided. Private IP addresses and NAT are implemented within most home and business networks.

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9  Using networks securely


  • You need to consider the security of your home network in order to protect your personal security. If your network is not secured, or you use it in an insecure way, then you increase the risk of threats to yourself and your data.

  • Consider some of the activities that you undertake via the Internet:

    • emailing

    • sending photographs

    • chatting

    • posting to wiki

    • shopping

    • banking.

  • What would be the impact if someone was able to intercept any information you sent while doing these activities? It could range from personal embarrassment to severe financial loss. You would probably not do any of these things with a stranger sat next to you, but when you do them over the Internet, there is a risk that your data can be intercepted by others.

  • What could be the motivation for trying to access your information? The list is long, but some popular reasons are:

    • general nosiness

    • a wish to bully or get you into trouble

    • a wish to steal your money or data

    • a desire to impersonate you while carrying out a criminal activity.

  • To start securing your network, consider the passwords that you use to protect access to your online accounts, and to your network devices such as PCs, tablets and home routers.

    • Your passwords need to be strong. Use a mix of numbers, letters and special characters. Do not use anything that could be guessed, such as a pet’s name or a birthday.

    • You need to change your passwords regularly in case other people learn them.

    • Do not tell your passwords to anyone. Ever.

    • If you have set up a password hint, do not make the hint too obvious.

    • Do not use the same password across multiple accounts and devices. If someone learns it, they have access to everything.

    • Do not write your passwords down.

  • You also need to consider how your devices handle your passwords, as some will try to be helpful and store them for you. If you are using a public computer to access a website that requires you to enter a password, the browser may offer to store it for you. This is not a good idea, as the next user of the computer can access your passwords.

  • If you log into your account and then walk off to do something else, leaving the computer unattended, anyone else who is present can access your account. If you leave your PC or device, logout from it first. Most PCs, tablets and smartphones can be configured to automatically logout after a short period of inactivity, so find out how to set this up.

10  Malware


  • Malware is a type of computer program created by criminals with the intention of stealing or damaging data and perhaps disrupting network operation. There are three main types of malware:

    1. Virus: malicious software attached to another program to execute a particular action on a computer. Viruses normally require the intervention of humans in order to propagate themselves, and are commonly received as attachments to emails or as files stored on USB memory sticks.

    2. Worms: self-contained malware programs that attack a computer and try to exploit a specific security ‘hole’ or vulnerability in a software program installed on it. Once they have successfully attacked the vulnerability, the worm copies its program across the network to attack other devices on the network.

    3. Trojan horse: similar in operation to a worm, except it is disguised to look like a useful software program that you may want to install on your computer. Once you have installed the Trojan, it will act as if it is normal software, but will be secretly carrying out some criminal activity such as logging the keys you are typing in an attempt to copy your passwords. Trojans are normally capable of transmitting the information they steal to interested criminals via your network connection to the Internet.

  • There are many precautions you can take to protect yourself from malware:

    • Always install antivirus software on your computers and make sure it is kept up-to-date.

    • Always keep your operating system updated, as updates include patches for any vulnerabilities that may be exploited by malware. Most operating systems can be configured to do this automatically for you.

    • Never open email attachments from people you do not know. Always scan email attachments from people you do know.

    • Always scan your USB memory sticks with your antivirus software after inserting them into your computer. Never plug in a memory stick that you have ‘found’.

    • Always keep copies (backups) of your important files on a separate hard drive, preferably one that is not kept connected to your network. You can then retrieve your data if malware damages the originals.

11  Phishing


  • Many criminals will try to get you to reveal passwords and other account information by pretending to be someone else. This shouldn’t be an issue if you follow the advice already covered in this course – in particular that you should never share your passwords. However, some criminals are very skilled, and send emails and texts that appear to come from legitimate sources, such as a bank or a government agency:

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Figure 26

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  • These messages will normally direct you to a website, which may appear very convincing but is designed to capture all your login information. To protect yourself from phishing, remember that legitimate organisations do not conduct sensitive business via email or text – they will generally use the telephone or send a letter. When they do email, it will be a request for you to login to your account securely, so be suspicious of any links to websites that could be run by fraudsters.

  • Always check the address bar of your web browser to see which application layer protocol is being used to send your sensitive information to a website. Responsible organisations will use HTTPS, which is a secure version of HTTP and can prevent your data being intercepted by Internet-based criminals:

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Figure 27

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12  Activities


Start of Activity

Activity: Investigating your home network

Start of Question

Make sure you have your parent or carer’s permission to investigate this at home. If you are doing this at school, make sure you have permission from a teacher or IT Technician.



  1. What devices are wired on your LAN via UTP cable?

End of Question

Provide your answer...

Start of Question


  1. Does the cable go into an Ethernet switch or into sockets and disappear?

End of Question

Provide your answer...

Start of Question


  1. Where is the switch located?

End of Question

Provide your answer...

Start of Question


  1. How do you know if it is an Ethernet switch?

End of Question

Provide your answer...

Start of Question


  1. Do you have any wireless devices connected?

End of Question

Provide your answer...

Start of Question


  1. Where is the wireless access point?

End of Question

Provide your answer...

Start of Question


  1. How does your network connect to the Internet?

End of Question

Provide your answer...

Start of Question


  1. What type of WAN connection does your network have and what connectors does it use?

End of Question

Provide your answer...

Start of Question


  1. Is there a modem and, if yes, where is it situated?

End of Question

Provide your answer...

Start of Question


  1. If at home can you find the address given to your router from the ISP? (It is called your ‘public’ address.)

End of Question

Provide your answer...

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  1. How much you have learnt about networking?

End of Question

Provide your answer...

End of Activity

Start of Activity

Activity: Network devices and protocols – Packet Tracer

Start of Question

We connect our devices to the Internet and the Internet connects them all together. But what is the Internet made of? We say it interconnects all the devices, but how?

Network devices are mainly hubs, switches or routers. There are lots of other specialist bits, but the bulk of the interconnectivity is done by switches and routers. In this course you have looked at client server models, web requests and emails. What do these messages look like and how are they sent?

For each type of communication over the network there are a set of rules and specifications as to what format and order the information is sent. These are called protocols and they are fundamental in understanding how the Internet works.

This Packet Tracer lab explores the devices and protocols used in computer networks. You will need:



  • Lab Book: Identify Devices and Protocols Used in Computer Networks

  • Packet Tracer

  • CASBIT.pkz

End of Question

End of Activity

Start of Activity

Activity: Password hints

Start of Question

Companies do not store a user’s passwords in clear text – they send it through a scrambling algorithm to produce a 'hash', and the hash is then saved. The hash algorithm does not work in reverse, so you can't unscramble a hash to get the original password. When you enter your password the site uses the same algorithm to make a hash, which it then compares to the hash saved against your details.

Some sites allow you to store password hints, and they save these as clear text. Adobe was hacked in 2013, and its hashed passwords and hints were stolen for thousands of customers. The hackers looked at all the hints that gave the same hash (hence were the same password). Because they had lots of hints for the same password it was easy to guess what they were.

Can you guess the passwords used in this puzzle based on the stolen hints?

End of Question

End of Activity


13  Resources


For more information, take a look at the following resources.

  • Watch this computer networking tutorial to explore what is a protocol:

Start of Media Content

Watch the video at YouTube.com.

End of Media Content


  • For a detailed look at the complexity of what happens when you click a URL read Igor Ostrovsky’s blog post.

  • Read Google's guide to online safety.

  • Read How-To Geek’s guide to choosing a good password and watch the following video:

Start of Media Content

Watch the video at YouTube.com.

End of Media Content


  • Read Action Fraud’s guide to phishing and watch the following video:

Start of Media Content

Watch the video at YouTube.com.

End of Media Content


  • Watch this video on being cautious when using public WiFi:

Start of Media Content

Watch the video at YouTube.com.

End of Media Content

14  End of course quiz


Now it’s time to test what you’ve learned in a quiz.

15  Acknowledgements


Grateful acknowledgement is made to the following sources:

Figure 1: Birmingham City University (BCU)

Figure 2: Birmingham City University (BCU)

Figure 3: Cisco

Figure 4: Birmingham City University (BCU)

Figure 5: Birmingham City University (BCU)

Figure 6: CommScope. This file is licensed under the Creative Commons Attribution-Noncommercial-NoDerivatives Licence http://creativecommons.org/licenses/by-nc-nd/2.0/

Figure 7: tlsmith1000. This file is licensed under the Creative Commons Attribution-Share Alike Licence http://creativecommons.org/licenses/by-sa/2.0/

Figure 8: Bull3t Hughes. This file is licensed under the Creative Commons Attribution-Share Alike Licence http://creativecommons.org/licenses/by-sa/2.0/

Figure 9: Cisco

Figure 10: Cisco/Birmingham City University (BCU)

Figure 11: Cisco/Birmingham City University (BCU)

Figure 12: Cisco/Birmingham City University (BCU)

Figure 13: Cisco/Birmingham City University (BCU)

Figure 14: Cisco/Birmingham City University (BCU)

Figure 15: Cisco

Figure 16: Cisco/Birmingham City University (BCU)

Figure 17: Cisco/Birmingham City University (BCU)

Figure 18: Cisco/Birmingham City University (BCU)

Figure 19: Publisher unknown

Figure 20: Cisco/BCU

Figure 21: Birmingham City University (BCU)

Figure 22: Birmingham City University (BCU)

Figure 23: Birmingham City University (BCU)

Figure 24: Birmingham City University (BCU)

Figure 25: Birmingham City University (BCU)

Figure 26: Publisher unknown

Figure 27: Screenshot taken from Barclays, example of phishing.

Every effort has been made to contact copyright holders. If any have been inadvertently overlooked the publishers will be pleased to make the necessary arrangements at the first opportunity.



Page of 29th August 2017

http://www.open.edu/openlearncreate/course/view.php?id=2772

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