ManojK's Resource Centre

Computer Networks

One of the most useful things about computers is their ability to ‘talk’ to each other, when two or more computers are connected together, we call this a network. 

A computer network is a number of computers linked together to allow them to share resources.  

Benefits:

·         Programs can be shared - software packages can be installed onto the file server and accessed by all individual workstations at the same time. This reduces cost, maintenance and makes upgrades easier.

·         You can access your work from any workstation on the network. Very handy if you have to change computer every time you go to a different classroom.

·         Data can be shared by all users at the same time. Many people can access or update the information held on a database at the same time. Thus information is up to date and accurate. Users can communicate with others on the network by sending messages and sharing files.  

Disadvantages:

·         Networks can be expensive to set up. They often involve taking up floors and ceilings to lay hundreds of feet of cables

·         Networks are vulnerable to security problems. Hackers, disgruntled employees or even competitors might try to break into the system to read or damage crucial information. Much effort is spent preventing unauthorized access to data and software.

·         If the main File Server breaks down, then the whole system becomes useless and no one can carry on working. Malware, such as worms, easily spread on computer networks and not on standalone computers.

·         Because networks are often complicated, they need expensive expert staff to look after them.  

Types of Networks:

LAN- Local Area Network

SAN- Small Area Network

MAN- Metropolitan Area Network, and

WAN- Wide Area Network

It is this ability of different computers to share information which has made the INTERNET possible. The INTERNET is a Wide Area Network.  

Local Area Network:

This include network where the computers are relatively close together. So LANs would be within the same room, a single building or several buildings close together.  

Local Area Network-Important:

LANs use dedicated cables or wireless technology. Each computer connected to a LAN requires a special hardware

·         If the computer is to be connected to a wireless network it will need a wireless adapter.

·         If the computer is to be connected using cables, then a network PCI card is required.  

Categories of Computer Network:

·         Peer-to-Peer Networks

·         Client - Server Networks

Peer-to-Peer Networks:

Peer-to-Peer Networks A peer-to-peer network allows two or more PCs to pool their resources together. Individual resources like disk drives, CD-ROM drives, and even printers are transformed into shared, collective resources that are accessible from every PC.

The advantages of peer-to-peer are:

·         No need for a network administrator

·         Network is fast/inexpensive to setup & maintain

·         Each PC can make backup copies of its data to other PCs for security. 

·         Easiest type of network to build, peer-to-peer is perfect for both home and office use.  

Client - Server Networks:

In the client/server network model a computer plays a centralized role and is known as a server all other computers in the network are known as clients. All client computers access the server simultaneously for files, database, docs, spreadsheets, web pages and resources like hard diver, printer, fax modem, CD/DVD ROM and others. In other words, all the client computes depends on the server and if server fails to respond or crash then networking/communication between the server and the client computes stops  

Client - Server Networks:

Useful in centralization of data

Server- A computer or program which provides services (files, security clearance, access to hardware, etc.) requested by client, Computers or programs over a network.

Client- A computer or program that requests a service of another computer system or program (the server).  

Network Topologies:

Physical Structure of the Computer Network

·         Star

·         Ring

·         Bus

Star Topology:

The star pattern connects everything to one server, which is the computer that handles the network tasks and holds the data. All communications between computers go through the server. This configuration is good for businesses that have large amounts of rapidly changing data, like banks and airline reservation offices.  

Advantages

·         Gives close control of data.

·         Each PC sees all the data.

·         User sees up-to-date data always.

·         If a computer other than the host fails, no other computer is affected.  

Disadvantages

If host computer or its software goes down, the whole network is down. (A backup computer system would be necessary to keep going while repairs are made.)  

Ring Topology:

The ring pattern connects the computers and other devices one to the other in a circle. A central server is not required. Communication flows in one direction around the ring. This configuration is good when the processing of data can be done on the local PC.  

Advantages

Requires less cabling and so is less expensive.

Disadvantages

If one node goes down, it takes down the whole network.  

Bus Topology:

The bus pattern connects all the workstations to the same communications line. Communications travels both directions along the line. All the workstations can communicate with each other without having to go through the server.  

Advantages

Any one computer or device being down does not affect the others  

Disadvantages

Can't connect a large number of computers this way. It's physically difficult to run the one communications line over a whole building, for example.

Wide area Network:

·         A computer network that crosses geographical boundaries

·         Many small WAN’s can make up a larger WAN

·         The Internet is the biggest WAN as it is comprised of all of the smaller

·         WAN’s and LAN’s in the world connected to the Internet

·         Uses Fiber Optic Cable, Satellite technology, existing TV cable phone lines to send data over the network

Wireless Networks:

·         Wi-Fi -> wireless fidelity

·         Based on the IEEE (Institute of Electrical and Electronics Engineers) 802.11 standard

·         Wi-Fi is a wireless technology that uses radio frequency to transmit data through the air.  

·         A wireless network is any computer network where there is no physical wired connection between sender and receiver, but rather the network is connected by radio waves and/or microwaves to maintain communications.

A hotspot is a public area where users can access the Internet using wireless local area networks (WLAN).  

What do you need to do Wi-Fi? :

·         Existing wired network / services (Infrastructure Mode) DHCP/DNS

·         Access point

·         Antennas and bridges

·         Wireless adapter

How does Wi-Fi work? :

Wi-Fi, wireless fidelity, is the wireless technology to handle the networking/communication.

Wi-Fi allocates internet / intranet connection globally and to be transmitted by the radio waves. Radio waves are the main cause of Wi-Fi. Radio waves are transmitted from antenna and Wi-Fi receivers pick them up. When a user receives the Wi-Fi signals, a wireless internet / intranet connection is produced and a user is prompted to provide the user name and password if required to establish a wireless connection.  

Wireless Networks-ADVANTAGES:

·         Cheaper to set up because you don’t have to buy and install network cable

·         Setup Cost – Reduced cabling required

·         Flexibility – Quick and easy to setup in temp or permanent space

·         Scaleable – Can be expanded with growth

·         Freedom – You can work from any location that you can get a signal

·         Lower total cost of ownership – Because of affordability and low Install cost

·         Allows mobility with laptops, PDA’s smart phones, and PSP’s  

Wireless Networks-DISADVANTAGES:

·         Planning – Depending on the goal

·         Security – Greater exposure to risks

1.      Access

2.      Compromising Data

3.      Denial of Service

·         Speed – Slower than cable

·         Range – Affected by various medium

1.      Travels best through open space

2.      Reduced by walls, glass, water, etc  

Types of security ~~ Wi-Fi:

Wired Equivalency Privacy (WEP) - WEP encrypts data transmitted over the WLAN

·         A 64-bit WEP encryption is almost always entered by users as a string of 10 Hexadecimal (Hex) characters (0-9 and A-F)

·         A 128-bit WEP encryption is almost always entered by users as a string of 26 Hexadecimal (Hex) characters (0-9 and A-F)

·         128 bit encryption is more secure because it takes longer to break the passcode because of its increased number of characters.  

WiFi Protected Access (WPA) - WPA-PSK is an extra-strong encryption where encryption keys are automatically changed (called rekeying) and authenticated between devices after a specified period of time, or after a specified number of packets has been transmitted. This is called the rekey interval. WPA-PSK is far superior to WEP and provides stronger protection for the home/SOHO user for two reasons. The process used to generate the encryption key is very rigorous and the rekeying (or key changing) is done very quickly. This stops even the most determined hacker from gathering enough data to break the encryption.  

Media Access Control (MAC) address filtering - a hardware address that uniquely identifies each node of a network you can identify which nodes have permission to access a network by their individual MAC address.  

Secure Your Wireless Network:

·         Encrypt your traffic

·         Don’t broadcast your SSID.

·         Filter MAC address.

·         Change the default SSID, administrator account, and password.

·         Don’t use an SSID that makes sense to other people

·         Check your antennae

·         Consider AP placement.  

Network types:

Internet - The Internet was created back in 1969, during the Cold War, by the United States military. It was meant to be a "nuke-proof" communications network. Today, the Internet spreads across the globe and consists of countless networks and computers, allowing millions of people to share information. Data that travels long distances on the Internet is transferred on huge lines known collectively as the Internet backbone. The Internet is now maintained by the major Internet service providers such as MCI Worldcom, Sprint, GTE, ANS, and UUNET. Because these providers make huge amounts of revenue off the Internet, they are motivated to maintain consistent and fast connections which benefits everyday Internet users like you and me.  

Intranet - Intranet is an internal or private network using Internet technology that can only be accessed within the confines of a company, university, or organization

Extranet - An extranet is a private network that uses Internet technology and the public telecommunication system to securely share part of a business's information or operations with suppliers, vendors, partners, customers, or other businesses. An extranet can be viewed as part of a company's intranet that is extended to users outside the company.

Virtual Private Network (VPN) - A VPN refers to a network that is connected to the Internet, but uses encryption to scramble all the data sent through the Internet so the entire network is "virtually" private. 40  

Network types-IMPORTANT:

The Internet and the World Wide Web are not the same thing. The World Wide Web is the Internet feature that allows you to visit web pages from all over the world. It is one of the many features of the Internet. E-mail, FTP, and Instant Messaging are also features of the Internet.

Security:

The main cause of a security threat is the misuse of the internet without proper anti-virus, anti-spyware. Other causes are:

·         Insiders

·         Network Intrusions

·         Malware and Adware T

·         rojan Horses, Worm

·         E-mail Attachments

·         Floppy Disk, CD and Pen Drive , or

·         Any Infected Computer That Is Attached To The Network.

A networks greatest strength the ability to share data is also its greatest weakness.  

Network Security Issues:

·         Who has access to the network?

·         Is anti-virus and anti-spyware software installed on all client computers and is it kept up-to-date? Is a Firewall used and is it configured appropriately for the network it protects?

·         Is there a network policy that specifically outlines what is and what is not allowed to be done on the network by the network users?  

Encryption:

The scrambling of data into unintelligible characters using a key (usually some sort of mathematical transformation). The encrypted file can be restored to its original state (decrypted) using the correct key. Modern encryption methods involve the use of one-way functions (easy to encrypt, very difficult to decrypt) . Such asymmetric encryption involves the sender encrypting with a recipient's public key, which can then only be decrypted by the recipient themselves (the only one with access to the private key).

Secure Sockets Layer (SSL):

Secure Sockets Layer (SSL) a protocol, originally by Netscape, designed to give secure communications on the Internet. SSL operates below the application protocols (HTTP, Telnet, FTP, Gopher, etc.) but is layered above the basic TCP/IP protocols so it "protects" the applications that are transmitted over the TCP/IP connection.  

URL- Uniform Resource Locator:

URL- Uniform Resource Locator Uniform Resource Locator, address on the World Wide Web e.g. http://www.kasigaschool.com

Hyperlink :

Hyperlink Links or references within Web documents that upon selection (clicking the mouse with the cursor located over the document link) cause jumps to another location within the document, to other documents, or to programs that process images, sound, videos, operate upon databases, or perform other functions. Hyperlink objects may be words, phrases, images, or parts of images. Hyperlinked objects are usually displayed in a manner to distinguish them as links. Image maps may not show obvious linked portions and many areas of an image can be linked to different targets.  

Webpage and Website:

Webpage is a document on the World Wide Web. These documents are used with browsers to display text and images and to play sound, video, or animation programs. Web page files contain HTML codes to control the display and playing of their associated components. Website is the collection of webpage.

Web Browser:

Web Browser is application software used to browse i.e. used to locate and display Web pages.

Protocols:

A high level of coordination is required for the exchange of information between computers on a network. This is provided by a set of rules for information exchange that must be adhered to, by all computers on the network. This set of rules is called a protocol.

Protocol is format-rules and regulations, for transmitting data between two devices.

File Transfer Protocol (FTP): A protocol used between clients and servers allowing one computer to transfer files to and from another computer over a TCP/IP network. Connections require a user identification which may be open to "guest" or "anonymous" users, or the connections may be restricted to specific persons. The connections may require public passwords, e-mail addresses or secret passwords. Users may be permitted only to download files or they may be given permission to both upload and download files.  

Hypertext Transfer Protocol (HTTP): The client-server TCP/IP protocol used on the World-Wide Web for the exchange of HTML documents. Hypertext: A document or set of documents with "links" that aid users in navigating among links and their references. Hypermedia: Like hypertext but includes graphics, sound, video and other kinds of data.  

Hypertext: A document or set of documents with "links" that aid users in navigating among links and their references.

Hypermedia: Like hypertext but includes graphics, sound, video and other kinds of data.

TCP/IP: TCP/IP stands for Transmission Control Protocol / Internet Protocol. TCP is for communication between applications while IP is for communication between computers. TCP/IP defines the rule computers must follow to communicate with each other over the internet.

Cookies :

A small file used in communication between cooperating programs. Cookies are used by some browsers and Web server programs to identify the client user and even unique preferences or requests from the client user e.g. their user name and password. Cookies may be stored for use during a given session, for a set length of time (seconds, minutes, hours, or days) or retained permanently. Cookie information is stored with the browser on the client side; the information is automatically accessed and used by the browser in subsequent transactions, often without the user's knowledge.

Listserv :

An electronic discussion support system. Users can subscribe and unsubscribe by e-mail. All messages sent to the discussion list are automatically sent to all subscribers. Most list servers support archiving of the messages by day, week, month, or year.  

Webcams:

Webcam is real time camera whose images can be accessed using the WWW, instant messaging or video conferencing software. It will be a digital camera and will upload its images to a web server, either continuously or at regular intervals. Web server is a computer program that receives and processes requests from client browsers.

Search engines and their appropriate use:

Web crawler is a specific search engine developed by Brian Pinkerton at the University of Washington. It is a freeware program that "roams” the World Wide Web and collects URLs. Users can then perform searches by entering keywords.

Points to Remember

Computer Networks