Computer Science
The Internet

The term 'Internet' comes from the terms, 'Internetwork' & 'Internetworking'. These terms are used to describe a connection between 2 or more networks. Any network of computer networks is an internet or internetwork.

The Internet (definite article, capitalisation) is the 'special' internet that is available to the general public.

The Internet is a wide area network (WAN) - its connections span a geographically wide area.

The World Wide Web

The web began in 1989 in CERN, the idea of Tim Berners-Lee to link together scientific data and allow easy navigation between resources.

Tim Berners-Lee wrote the first web browser in Pascal. The term now refers to the documents that we access via the Internet.


An Intranet is what you get when you provide Internet-like services within an organisation. It uses the same protocols as the Internet and the experience of using it is similar. At school, an Intranet is used to publish a number of web pages. It is relatively easy to do this on a home network if you install a web server.

The Networks

The following diagram describes the way that the networks that make up the Internet are connected.

internet diagram

The Internet backbone is made up of many large networks which are connected to one another. These large networks are known as Network Service Providers or NSPs. These networks exchange packet traffic. Each NSP is required to connect to three Network Access Points or NAPs. At the NAPs, packet traffic may jump from one NSP's backbone to another NSP's backbone. NSPs also interconnect at Metropolitan Area Exchanges or MAEs. MAEs are privately owned but perform the same function as the NAPs. NAPs were the original Internet interconnect points. Both NAPs and MAEs are referred to as Internet Exchange Points or IXs. NSPs also sell bandwidth to smaller networks like ISPs.


Data is sent across networks in manageable chunks called packets. Sometimes packets are known as datagrams. The Internet is based on a technology called packet switching.

Packet (Simple Version)

A packet consists of

  • Source address
  • Destination address
  • Data to transport

Circuit Switching

Circuit switching is used in the public telephone system. When a a call is made, a path is set up between the caller and receiver and is maintained for the duration of the call. In a network, a circuit can be established between two devices transmitting data at the same rate. Because data segments arrive in the order that they are sent, less processing time is required to reconstruct messages when they are received.

Search for an animation that demonstrates this - there are many on the WWW.

Packet Switching

Before a message is sent, it is divided up into packets. Each packet is then transmitted individually and can even follow different routes to its destination. Once all the packets forming a message arrive at the destination, they are recompiled into the original message.


The information used to get packets to their destinations is contained in routing tables kept by each router connected to the Internet. Routers are packet switches. A router is usually connected between networks to route packets between them. Each router knows about it's sub-networks and which IP addresses they use. The router usually doesn't know what IP addresses are 'above' it.

router hierarchy

The black boxes connecting the backbones are routers. The larger NSP backbones at the top are connected at a NAP. Under them are several sub-networks, and under them, more sub-networks.

When a packet arrives at a router, the router checks the destination address that wit was given when it was created. Then the router checks its routing table. If it has the address of the network that contains the destination address, the packet is sent to that network. If it doesn't have the address of the network that contains the IP address in the packet, it sends the packet on a predefined default route, usually up the hierarchy to the next router.

The next router may know the address of the network the packet is heading for. If it does not, it routes the packet up the hierarchy again until either the packet gets sent towards its destination or reaches an NSP.

The routers connected to the NSP backbones hold the largest routing tables. The packet will now be routed towards the correct backbone and will find its way back down the hierarchy to its destination.

The End-To-End Principle

When two computers communicate on the Internet, they are in control of the communication. The role of network is to move packets between the two points. This approach has advantages.

The routers do not have to know if endpoints are connected. The endpoints handle this. This simplifies the design of the router. The role of the Internet is to move packets, the endpoints handle all the details of maintaining security and integrity of information. Endpoints connect to a single router.

Open Architecture

The principles of operation described so far, describe an open architecture network. This term refers to the fact that any network can be connected to the Internet via a router gateway.