COMPUTER VIRUSES

One might think of a computer virus as a tiny computer program designed to perform mischief. Most computer users have heard about computer viruses. A computer virus is the result of a destructive program that someone has written and placed inside a computer program, which unsuspecting people then place in their computer system. Some viruses can erase all the information from the place where it's stored on the computer's hard disk. But each virus is different. Some display strange messages on your computer screen; others make small changes in your computer programs.
Some people create these programs out of meanness to get even. While others create them just as a challenge. Why do you thing people create these very destructive programs? How does your computer get a virus? Almost exactly the way humans do. The computer gets exposed to one. Well, its not quiet that easy. Many people get contaminated computer programs by trading programs with other people. Others get contaminated computer programs through the use of modems, which allow computers to communicate over telephone lines.
Most of the time, programs that arrive by modem or a trade are perfectly safe to use. However, you do stand a chance of getting a program that has been tampered with. Here a computer program virus is hiding inside the normal program. Many computer programs that are traded were copied illegally. When this program enters your computer through your input device, it hides in your computer's memory and starts to duplicate itself like a disease. When you save your data, you also save the virus. Slowly but surely, the virus crowds out your data and causes major system problems.
The virus can't affect the computer's ROM (Read Only Memory), but it can affect RAM (Random Access Memory) and your computer disks. When your shut off your computer a virus that has been picked up will be lost, just like any other memory that is held in RAM. If the virus is on your disk or hard drive, it will return to the computer when you use the program again. If you switch from one program to another without shutting down the machine, the virus will attach itself to the new program. In this way, it can slowly infect all your programs before you know that it exists. Today millions of dollars are being spent to rid and protect computer systems from these virus programs.
Commercial and shareware programs have been created with the sole purpose of detecting and fixing suspect programs that might be viruses infected. These detection programs should be ran when any disk is put into your disk drive or every time your computer is first started up each day to scan the computer's hard drive.

Computer Network

A computer network may be defined as the coordination or interconnection of a number of individual computers. A computer network is basically established by the network layer in the Open Systems Infrastructure model, popularly known as the OSI model. Computer networks can be classified by means of a lot of types like the network size, structure, mode of connectivity, topology and others. Some of them are.
Considering the size of the implemented network, Computer networks are classified into 3 main divisions namely Local Area Network (LAN), Wide Area Network (WAN) and Metropolitan Area Network (MAN). .
Clasifications of computer network :

LAN: - A network of computers which cover only a small area like an office or a building can be referred to as a Local Area Network. All the individual computers must have a common protocol layer for their connectivity with the basic functionality taking place at the network layer. In general, LANs are based on Ethernet technology.

WAN: Wide Area Network is basically an extension of the Local Area Network except for the fact that the size of the network extends to a very large area. The major functionality of the WAN networks take place at the 3 lower layers of the OSI model namely network layer, data link layer and the physical layer.

MAN: Metropolitan Area network is a combination of two or more individual Local Area Networks but with a small criterion of the boundary of the network not exceeding the city limits, thus integrating the network as a single unit within a city.