Chapter 4: Computer Networks Revision

Computer Networks What is a Network?

What is a Computer Network?A network is two or more computers, or other electronic devices, connected together so that they can exchange data.

For example a network allows:

• Computers to share files, • Users to message each other,• Share Resources

Network connections between computers are typically created using cables (wires).

However, connections can be created using radio signals(wireless / wi-fi), telephone lines (and modems) or even, for very long distances, via satellite links.

wireless access point

RouterISP

Server

Firewall

Bridge

Computer Networks Overview of Network Devices

Setting up a Network:

• Switch to connect Network Devices together using cables

• A separate wireless access point - to allow wireless devices (e.g. laptops or smart-phones) to join the network

• A router to connect your LAN to the Internet (WAN)

• A firewall to protect your network from hackers

• A Bridge to connects two parts of a Local Area Network together

• Server(s) to manage network functions such as network security, network file storage (central), shared resources (such as printers).

• Set up an account with an Internet Service Provider (ISP) to receive an internet connection to your location

Computer Networks Common Network Devices

Router • Connects network/computers to the internet• Connects LANs/networks together• Transfers data between networks (Receives and Sends Data Packets)• Router can connect to devices using cables or wireless signals. • It stores information about which computer is connected to which network

Hubs • A hub is a device that connects a number of computers together to make a LAN. • A hub is a ‘dumb’ device: if it receives a message, it sends it to every computer

on the network. • This means that hub-based networks are not very secure.Normally used in small home based networks.

Switches • A switch, like a hub, is a device that connects a number of computers together to make a LAN.

• A switch is a more ‘intelligent’ device than a hub: if it receives a message, it checks who it is addressed to, and only sends it to that specific computer.

• Because of this, networks that use switches are more secure than those that use hubs, but also a little more expensive

Normally used in larger networks found in schools, offices etc.

Bridges • A bridge is a network device that typically links together two different parts of a LAN.

Computer Networks Using a computer connected to a network allows us to…

Advantages of using Networks

• Easily share files and data• Share resources such as printers and Internet connections• Communicate with other network users (e-mail, instant

messaging, video-conferencing, etc.)• Store data centrally (using a file server) for ease of access

and back-up• Keep all of our settings centrally so we can use any

workstation

Disadvantages of using Networks

• Greater risk of hackers• Greater risk of viruses (spreading and disabling network)• The significant cost of extra equipment• When the network is down, cannot use network

computers without network connection.• Print queues can be long

Computer Networks Use of Router, Wi-Fi and Bluetooth

Wi-Fi

WiFi is a technology which makes it possible to connect a device to a network or to a single computer through wireless communications, for example:

• Laptops• Mobile Phones, PDAs and other handheld devices• Large computers which are WiFi enabled.

Bluetooth

Bluetooth is a wireless networking technology designed for very short-range connections (typically just a few metres).

Typical uses of Bluetooth: • Connecting wireless devices such as mouse, keyboard, headset to a

computer.• Transferring files between devices.• Printing Wirelessly from a PDA or Mobile Phone.

Advantages of Wireless Technology: • Reduced cost of cabling• Safer – won’t trip over wires• Easier to connect other devices to

the network• Makes the computer portable as long

as it’s within range of the wireless access point

Disadvantage of Wireless Technology: • Limited area of network• Strength of signal is weaker• Possible slow data transfer speeds• Easier to hack into/less secure• Physical obstacles can interfere with signal/can

cause disconnection

Computer Networks Types of Networks (LAN & WAN)

LAN• LAN is a Local Area Network• LAN covers a small area/one building• Often a LAN is a private network belonging to an organisation

or business.

Because LANs are geographically small, they usually use cables or low-power radio (wireless) for the connections.

WAN• A WAN is Wide Area Network is a network that extends over

a large area. • A WAN is often created by joining several LANs together, such

as when a business that has offices in different countries links the office LANs together.

Because WANs are often geographically spread over large areas and links between computers are over long distances, they often use optical fibre (glass) cables, satellite radio links, microwave radio links, etc.

Computer Networks WLAN – Wireless Local Area Network

WLAN

• A wireless LAN (WLAN) is a LAN that uses radio signals (WiFi) to connect computers instead of cables.

• At the centre of the WLAN is a wireless switch or router

• It is much more convenient to use wireless connections instead of running long wires all over a building.

However, WLANs are more difficult to make secure since other people can also try to connect to the wireless network.

So, it is very important to have a good, hard-to-guess password for the WLAN connections.

Computer Networks Common Networks – The Internet & Intranets

Internet• Internet is Public (available to all users)• Internet is network of networks • Internet is global• Internet has more information than an

intranet

Typical uses of an internet would be:

• Viewing web pages • Sending and receiving e-mail messages• Sharing files• Communicating using voice (VOIP)

and video (video-conferencing)• Playing multi-player games• Streaming Video/audio Content• Online Shopping/Banking

Intranet • Intranet is within one organisation (Private)• Intranets tend to be policed/managed• Intranet has an extra layer of security• Data found in an intranet is likely to be more

reliable/relevant than that found on the Internet Typical uses of an intranet would be:

• Viewing internal web pages (e.g. company schools, university's etc.)

• Internal e-mail and instant-messaging between workers

• Sharing of internal documents

Computer Networks Network and Data Security: Proxy Server

Proxy Server

Server passes on requests to the InternetPasses the requested web pages to individual computers

Can be used to monitor Internet usage

Can cache/store the webpages

Can block certain sites

Can act as a buffer (between Internet and LAN)

Can act as a web server

Computer Networks Network and Data Security: Firewall

Monitors and controls incoming and outgoing data traffic

Helps to prevent malware (viruses) getting into computer/from the internet

Prevents computer accessing undesirable/unauthorised sites

Warns you regarding threats/allows you to accept/reject downloaded programs

Keeps a list of undesirable sites/IP addressesKeeps a list of acceptable sites/IP addresses

Prevents unauthorised computers using the internet accessing the computer

Computer Networks Network and Data Security: Data Encryption

Disadvantages

• Data can still be deleted from system• Criminals can use encryption to keep incriminating material secure

Encryption is the process of converting information into a form that is meaningless to anyone except holders of a ‘key’.

1. Data is scrambled up (Data is Encrypted).

2. If the data is intercepted it then it prevents people from understanding and reading the data.

3. The person with the key is able to Decrypt and understand the data.

Computer Networks Network and Data Security

Methods of Authentication (Network Security)

User Name & Password

• User ID: When logging onto any network system, a user will be asked to type in a user ID. This will be unique to the user. A user ID could be made up of letters and numbers

• Passwords: When the user has entered their user ID into the login they will then be asked to type in their password. This will appear at asterisks (******).

• If the password does not match up with the user ID then then login will be rejected.

Biometrics

• Finger prints, retina, face, voice can be used to gain entry to a system.

• Main advantage is the data will be unique and hard to replicate.

Magnetic Swipe Cards

• Will only allow people with the swipe card to gain access to the system.

• Swipe cards are quite easy to update.

• Disadvantages is the cards can be easily lost or replicated.

Computer Networks Network and Data Security

If your computer is connected to a network, other people can connect to your computer. A person who gains unauthorised access to a computer system is often called a hacker.

How can hackers effect your network and data?

• Introduce of viruses/malware/spyware to the network:• … which could result in data being deleted• Log key presses to get passwords to access the system

• Threat to Data:• Data could be deleted• Data could be replicated• Data could be corrupted• Data could be edited • Data could be sold or pass on to other people.

• Threat to Network:• Network may have to be shut down to quarantine virus.• This could stop production and cost an organisation a lot

of money.

Methods to keep Network & Data Secure other than authentication:

• Install Anti Virus

• Spyware

• Use of Firewall