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A computer can either be standalone or connected to a network. If it is standalone, then it can not communicate with any other computers. A computer connected to a network can communicate with one or more other computers. This communication may include, for example:

messages or emails

file transfers

application sharing

print sharing.

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A local area network (LAN) is usually confined within a local geographic area such as a building or campus. LANs are often found in schools, colleges, businesses and libraries. You will also find small LANs in private homes. A LAN has the following characteristics:

computers are within a local geographic area

the workstations have network cards or wireless connectivity

workstations and peripherals are connected via dedicated cables (or wireless devices) owned by the company

it often has shared peripherals (e.g. printers)

software and data can be shared.

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A wide area network (WAN) consists of computers that are geographically remote from each other. The characteristics of WAN are:

computers are not close together

external communications equipment, such as a telephone line, leased line or mobile phone are used to connect computers

a modem, router or other physical device is needed to connect the WAN.

The internet is the largest example of a WAN. Many computers are connected together through external communication links and information is shared via the World Wide Web and messages can be sent via email and internet relay chat.

In order to connect to the internet, an Internet Service Provider (ISP) is needed to provide the connection to the internet. An ISP offers services such as email, web hosting and file transfer.

WANs are also used in large organisations. Some examples include:

universities which have multiple campuses a long way from each other

local education authorities which connect together many schools

airline booking systems

National Lottery terminals

automated tell machines (ATMs)

train simetable systems

large businesses with many offices or shops.

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Most LANs are connected to a WAN. This means that if you are using a computer on a LAN, you will also be able to connect to other computers on a WAN. This is usually done using a router which is a device that connects the LAN to an external communication device that allows communication with a WAN (e.g. the internet).

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A virtual network allows computers to communicate with each other as if they were within a single LAN, but without knowing that they might be part of a bigger network.

A virtual network could exist within a single LAN so that a set of computers could communicate only with each other, even though there are other physical connections. In the figure, the green computers are part of a virtual network. They do not know that any of the grey computers exist on the network because the switching device that connects them all together is 'hiding' the other computers.

A virtual network could also exist across more than one LAN through a WAN or the internet. In in the image below, the green computers are part of a virtual network. They operate as if they were part of a single LAN. Other computers and devices cannot communicate directly with the green computers on the virtual network. Data for a virtual network travelling through cables on a LAN does not affect data from the LAN ot other virtual networks.

In a virtual network, users will have access to the same services they would on a single LAN, such as print sharing, file sharing and application sharing.