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Lesson 2Networks and the Internet
ObjectiveExplain the setup and Purpose of the Internet, Intranets, and Extranets.

Purpose of the Internet, Intranets, and Extranets

A network is a series of computers that are configured to communicate with each other, connected by one or more hubs and/or routers.
The Network and Internet layer supports the upper layer of the model by providing connections among linked computers, which are also called nodes or hosts.
On a small scale, a network can facilitate sharing of files between co-workers over a LAN (local area network). On a larger scale, an extranet can establish a secure area for remote customers to place and track orders or a company can use its intranet to post private, internal news that is instantly accessible to all employees world-wide. The networks and the Internet layer are illustrated in the diagram following Network Organization.

Network organization

Network configurations are broadly characterized in one of three ways: Internet, intranet, and extranet. Each of these has specific purposes and uses.
Internet, intranet, and extranet

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The Internet is a public network that links millions of computers around the world. It is the world's largest network, and it provides the information pipelines for the World Wide Web, as well as for email and other Internet services.


An intranet is a private network that operates within an organization on a local area network (LAN). It is contained and kept secure by means of a firewall, passwords, and user permissions. Users access an intranet using the same Web browser they use to access the Internet.


An extranet is a private network that is accessible only to internal members of an organization and select external individuals, such as business partners or regular customers. Most organizations deploy all three kinds of networks. You should keep in mind that the three kinds of networks are not mutually exclusive. They are ways of arranging the network, providing security, and determining appropriate access for different groups of users to different sets of data and documents.
Network security is a crucial concern for many organizations. Access to and from an intranet or extranet can be controlled through the use of a firewall system. A firewall consists of hardware and software designed to protect the perimeter of a network. Firewalls can restrict access to certain Web sites, filter incoming and outgoing data transmissions, and maintain a traffic log.

Virtual private networks

Virtual private networks (VPNs) allow organizations to leverage the public Internet to replace costly dedicated data networks. In the past, organizations had to build their own wide area networks by installing and paying for costly leased data lines. For example, VPNs can be used to communicate between branch offices over the public Internet by using special software that encrypts and decrypts data transmissions to ensure secure communications.
In the next lesson, you will learn about the most common technical protocols for communications over networks: TCP, IP, HTTP, and FTP.
Question: Who are typical users of the Internet, a company's Intranet, and a company's extranet?
Answer: The Internet is publicly available, with some exception for password protected pages. A company's intranet is available to its employees either through password access or because they are on a Lan. A company's extranet may be available to customers, suppliers, contractors, or partners, who have password access.