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Lesson 4 The Internet, VPNs, intranets, and extranets
Objective Distinguish between the Internet, VPNs, intranets and extranets.

Internet, VPNs, Intranets, and Extranets


The Internet is a network of networks linking computers all over the world. The content on the Internet is located in independently owned computers called servers. Servers are linked to regional networks, which in turn connect to the core of the Internet. These points of connection are called network access points (NAPs). This image illustrates Internet topology.
This diagram illustrates Internet topology.
This diagram illustrates Internet topology.
There are many ways that a company can connect to the Internet. Many home users connect via PC modems that use twisted-pair telephone lines. For faster connections, businesses and a few home users use ISDN, T-1 lines, or T-3 lines. Regardless of speed, all lines to the Internet are routed through local access providers. Local access providers use the user's Internet access provider to connect to one of the Internet's access points.


As shown here, a Virtual Private Network or VPN is a private network that is configured within a public network. It is constructed by using public wires to connect individual computers in a simulated private network.

This illustrates a VPN's connections.
This illustrates a VPN's connections

A VPN enables IP traffic to travel securely over a public TCP/IP network by encrypting the traffic from one network to another. This encrypting technique is called tunneling. Tunneling ensures that only authorized users can access the network and that the data cannot be intercepted. Basically, this means that company employees can access their company's network from remote distances without risk to network security.
  1. Transmission Control Protocol/Internet Protocol: A communications protocol developed under contract from the U.S. Department of Defense to internetwork dissimilar systems. Invented by Vinton Cerf and Bob Kahn, this de facto UNIX standard is the protocol of the Internet and has become the global standard for communications. TCP provides transport functions, which ensures that the total amount of bytes sent is received correctly at the other end. IP provides the routing mechanism. TCP/IP is a routable protocol, which means that all messages contain not only the address of the destination station, but also the address of a destination network. This allows TCP/IP messages to be sent to multiple networks within an organization or around the world, hence its use in the Internet.


Intranets are essentially private Internets, or Internets inside of an organization. As this image shows, they enable organizations to use the Web as a convenient, low-cost channel for information sharing and distribution among an organization's employees and its other members.

This illustrates how an intranet works.
This illustrates how an intranet works

An intranet's Web sites look and act just like any Internet Web site. The main difference is that the firewall surrounding an intranet prevents unauthorized access.


A method for keeping a network secure. It can be implemented in a single router that filters out unwanted packets, or it may use a combination of technologies in routers and hosts. Firewalls are widely used to give users access to the Internet in a secure fashion as well as to separate a company's public Web server from its internal network. They are also used to keep internal network segments secure. For example, a research or accounting subnet might be vulnerable to snooping from within.


Extranets are like intranets insofar as they are networks that are not available to the general public (like the Internet). But whereas an intranet resides behind a firewall and is accessible only to people who are members of the same company or organization, an extranet provides accessibility to a predetermined set of outsiders. You can think of an extranet as an Intranet between organizational partners. This image illustrates an extranet.

This illustrates how an extranet works.
This illustrates how an extranet works

Extranets are becoming a popular means for business partners to exchange information.

Question: How are the Internet, intranets, and extranets similar and how do they differ?
Answer: Intranets and extranets use similar protocols and equipment as the Internet, as all are TCP/IP-based networks. But intranets and extranets are limited in their physical distribution and by their security hardware and software. The next lesson is about switches, bridges, and routers--the devices that interconnect the different kinds of networks.
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