1. Computer Networks
IGCSE ICT Section 4

2. Introduction Objective: To be acquainted with:
The definitions of networking
Network topology
Network peripherals, hardware and software

3. Network Definition
A network can be defined as two or more computers connected together in such a way that they can share resources.
The purpose of a network is to share resources.
A resource may be:
A file
A folder
A printer
A disk drive Or
just about anything else that exists on a computer.
A network is simply a collection of computers or other hardware devices that are connected together, either physically or logically, using special hardware and software, to allow them to exchange information and cooperate.
Networking is the term that describes the processes involved in designing, implementing, upgrading, managing and otherwise working with networks and network technologies.

4. Advantages of Networking
Computers can communicate and share data and files.
Storage facilities, and other hardware peripherals such as printers can be shared.
Data from all the computers can easily be backed up centrally.
Communication with other network users ( , instant messaging, video-conferencing, etc.) is possible.

5. Disadvantages of networking
A computer on a network is vulnerable to hackers
If the network breaks, many tasks become very difficult
Your computer can more easily be attacked by a virus
Installing a network is expensive as it involves purchasing of networking hardware such as Hubs, cables, network cards, bridges, routers etc.
Large networks require specialist to maintain them, which again involves time and money.

6. Computers on a network servers and clients (workstations).
Computers connected together to create a network fall into two categories:
servers and clients (workstations).
Client computers are the normal computers that people sit at and get their work done.
Servers are special, powerful computers that provide ‘services’ to the client computers on the network. These services might include:
Providing a central, common file storage area
Sharing hardware such as printers
Controlling who can or can’t have access the network
Sharing Internet connections

7. Servers There are different types of servers, e.g.
File servers, which allow users to save and load data/files
Application servers, which allow distribution of application software to each computer
Printer server, which ensures that printing on a network is done in a queue.
Proxy server, which is used as a buffer between WAN and LAN.

8. Types of Networks Local Area Network
A local area network (LAN) is a computer network covering a small geographical area, like a home, office, or small group of buildings, such as a school, or an airport.
Often a LAN is a private network belonging to an organization or business. 
Because LANs are geographically small, they usually use cables or low-power radio (wireless) for the connections.

9. Types of Network Wireless Local Area Network (WLAN)
A wireless LAN (WLAN) is a LAN that uses radio signals (WiFi) to connect computers instead of cables.
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.

10. Types of networks Wide Area Network (WAN)
A 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. 
The largest and most well known WAN is the Internet.

11. LAN, WAN and WLAN
WLAN
LAN
WAN

12. Sample Questions What is the difference between a LAN and a WAN ?
Give (a) one example of a LAN and
(b) one example of a WAN.
Describe two advantages that a network has over a set of stand-alone computer systems.
What is the purpose of (a) a file server and (b) a printer server.

13. NETWORK HARDWARE
In order for a computer to operate on a network, there are a range of different hardware components that are required.

14. 1. Network Interface Card (NIC)
Network Hardware
1. Network Interface Card (NIC)
Any computer that is to be connected to a network, needs to have a network interface card (NIC).
Modern computers have inbuilt NICs. However you can also add an expansion NIC card.
Most laptops have two inbuilt NICs; one for the wireless network, and another for the wired network.

15. 2. Network cables Network Hardware
They are used to connect different devices together.
Cables are still used in most networks, rather than using only wireless, because
they can carry much more data per second,
They are more secure (less open to hacking).

16. Network Hardware
3. Hubs
A hub is a hardware device that connects multiple computers together in a network and allows them to share data packets.
Each computer will be connected to a single 'port' on the hub.
The typical use of a hub is at the centre of a star network
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 - everyone can listen in to communications.
These days hubs are not common. They have been replaced by switches.

17. 4. Switches Network Hardware
A switch is a more advanced form of a hub.
It is a more ‘intelligent’ device than a hub
If it receives a message, it checks which computer 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,
Switches are a little more expensive.

18. 5. Routers Network Hardware
A router is a network device that connects together two or more networks.
It transfers data from between the networks in an intelligent way; i.e. it forwards data packets to their destination using the most efficient route.
A common use of a router is to join a LAN to the Internet (WAN).

19. 6. Bridges Network Hardware
A bridge is a network device that links together two different parts of a LAN.
It allows a large network to be segmented into two LANs to reduce traffic being passed around the network.
The linked LANs act as if it is one LAN.

20. 7. Proxy servers Network Hardware
A proxy server is a computer setup to share a resource, usually an Internet connection.
It keeps a copy of often-used web pages in its local 'cache'.
If a client computer requests for a web site then the proxy server checks to see if it has a copy - if it does, then that local page is sent to the client and there is no need to use up bandwidth calling for it over the internet
A proxy server can also act as a data filter, only allowing data packets that conform to company policy. E.g. blocking certain websites at certain times.

21. 8. Firewalls Network Hardwares
A firewall is a device, or a piece of software that is placed between a computer/LAN and the rest of the network to block unauthorized connections being made to your computer/LAN.
Normal data is allowed through the firewall (e.g. s or web pages) but other suspicious data is blocked.
If you wish to protect your LAN from hackers out on the Internet, you would place a firewall between the LAN and the Internet connection.