1. The internet AOU University Dr Radwan-Associate Lecturer
Block 3
The internet
AOU University
Dr Radwan-Associate Lecturer

2. Introduction
The internet has probably had more impact on our daily lives than any other technology emerging from the information age.
Most people, even the young and old, will have heard of and used the internet.
There are some technical features why the internet has become so successful. These include:
Packet technology: which has provided a flexible mechanism for carrying a wide range of different services over the same network.
Internet (IP) addresses: these have provided a universal addressing scheme.
Protocols: these have allowed many new services to be added to existing architecture.
It is the technology that underlies the operation of the internet that we will look at in this part of Block 3.
We will see how routers and switches forward packets, look at the structure of IP addresses, consider some of the protocols that help to manage addresses.

3. Routing and switching
The telephone system and the internet are both examples of networks.
Network is characterized by end devices (telephones and computers) and points where more than one link are joined together.
These points are called nodes and are the places where end devices are joined to a network, and are where equipment such as switches are placed so that different links can be connected together.
The links are cables or wireless connections.
Data is sent along a path through the network made up of a series of connected links.
Figure 1 gives a graphical representation of a communications network.

4. Routing and switching
Figure 1

5. Routing and switching
For a communications network, mechanisms are needed for finding a route to the correct destination and to join links together so that data can be sent to the destination.
These two mechanisms can be described as routing and switching.
The switching in a network is done by devices that direct data from one incoming link onto another outgoing link. This process is called forwarding.
You can think of a switch as a roundabout or a junction in a road: it provides a choice of direction for sending data towards the destination.

6. Routing and switching
Although telephone networks and computer networks can share links with each other, and telephone calls and s (for example) are often carried over the same networks, there are important differences in the way these networks connect end systems together.
They use different switching principles: telephone networks use a form of circuit switching, whereas computer networks use packet switching.
In a circuit-switched network, data is sent on the same end-to-end connection through all the intermediate nodes.
Once the path in a circuit-switched network has been established (a phone call is answered) it cannot carry data from any other sources until it is closed down (at the end of the call).

7. Routing and switching Packet switching:
Unlike a circuit-switched network, a packet-switched network does not establish a continuous path between end systems.
The data used to make up an , a video or even a phone call is broken down into small parts called segments.
Each segment is then carried as a payload within data envelopes called packets.
Each packet’s header contains data that identifies the destination and source addresses, and other data such as the type of data contained in the payload (see Figure 2 which shows a data packet).
Figure 2

8. Routing and switching
The internet is probably the best known example of a packet-switched network.
In this case, the address held in the header is called the IP address (Internet Protocol address) and a packet is referred to as an IP packet.
Each IP address is 32 bits in length and is normally written in a dotted decimal format. For example,
Local Area Network (LANs) also use the same packet-switching principles, but choices about forwarding are determined using Ethernet addresses, held in the header of the Ethernet packet.
Ethernet addresses are 48 bits long and are written normally in hexadecimal.
The devices that control packets using Ethernet addresses are actually called switches.
The devices using IP addresses are called routers.
Routers are commonly found in the internet and switches in LANs.

9. Routing and switching
Routers are network devices that are used to carry out packet switching in the internet.
Routers and switches use very similar principles for forwarding packets, as shown in Figure 3 which illustrates Packet switching.
At each of the nodes shown in the figure, the switch (or router) has to decide which output link to send any packet received on any input link.
For example, the second node from the left-hand computer has a choice of two links to send a packet towards the computer at the right-hand side
Figure 3

10. Routing and switching
Question: What information in the header of an IP packet is used to direct the packet to its destination?
Answer: Each packet header contains the destination IP address. The router uses this address to choose between the different links it has to other parts of the network, which allows the packet to eventually reach the correct destination.

11. Internet essentials
The internet is a public global internetwork made up of a huge number of networks, each of which can communicate with other networks.
Anyone who has a computer connected to the internet can connect to any other computer connected to the internet.
But how was this giant internetwork first developed?
The internet grew out of a 1969 research project that was carried out by the US government’s Advanced Research Project Agency (ARPA). Four locations only were linked together.
By 1980 there were only 15 universities connected to the ARPA network (ARPANET).
More academic networks were set up across the world and links between these networks began to appear.

12. Internet essentials
As the networks grew and became interconnected, this ‘network of networks’ became known as the internet.
Since that time, thousands of networks have joined the internet.
Organizations of all sorts (commercial, academic and governmental) have LANs on their locations, and many of them connect their LANs together to create an organization-wide network referred to as a wide area network (WAN).
The links that join different segments of networks – joining different LANs to create a WAN, for example – are described as backbones.
Lots of different networks of different sizes and shapes, all linked together to form the internet.

13. Internet essentials
When a network (LAN) joins to other networks (i.e. becomes part of the internet) – it has to conform to the standards of the internet.
This is what makes internetworking possible, and it is where the concept of a gateway comes in.
A gateway ensures that data from an individual network is compatible with the wider world of the internet.
When you use an internet service provider (ISP) to connect to the internet, for example, you are part of the ISP’s network, and the ISP has a gateway that connects you to the internet.
The internet is not owned by a single organization, so there is no one authority that dictates how it works.
All different people and organizations with their own networks (that together make up the internet) have to work to common standards, or data would be unable to move between the different networks.

14. IP packets How Internet messages are delivered?
Any internet message created by an application on your computer (an message or a request for a web page from your browser) is broken up into smaller pieces called segments.
The IP address of the destination computer is then added to each piece as part of a header, to make an IP packet.
This is sent onto the internet to be delivered to its destination via several networks and involving several routers and gateways between networks.
Each gateway puts another header on the packet, as well as adding more bits to the end (a ‘trailer’). This process is called encapsulation,
Encapsulation is done to make the packet suitable for the network that it is going through.
The extra header and trailer are removed when the packet leaves the network.
When the destination receives the packets from the source, it extracts the data from each packet (discarding the header), recombines the data and passes it to the appropriate application running on the destination computer.
The application software then interprets the message

15. IP packets
Protocol is the general term given to rules and formats that control the exchange of data between computers on a network.
A protocol involves splitting data up into packets and routing each packet using IP addresses.
It is the instructions that are used to create an IP packet (and break it down again) that make it a protocol.
Any computer that follows the protocol correctly can send and receive IP packets, even if the two computers are quite different , an iPhone and a PC, for example.
IP ,with some other important protocols, is often referred to as the TCP/IP family.

16. IP packets
Question: Why is the IP address so important when trying to find a destination computer?
Answer: Each computer connected to the internet can be uniquely identified by its IP address. This means that any computer can find any other computer if the IP address of the destination is known. Routers use IP addresses to forward packets towards their destination.

17. IP packets
The most important protocols for communication across the internet are the Internet Protocol (IP) and the Transmission Control Protocol (TCP), which are often spoken of together as TCP/IP.
IP has the job of getting data packets across the internet, while TCP is concerned with what happens at the sending and receiving devices.
Any computer connected to the internet is identified by its IP address which should be unique and is user in routing packets.
An IPv4 address is a set out as four numbers separated by dots.
Each of the four numbers is stored using one byte, and therefore can take any value between 0 and 255.
A byte is 8 bits, which as you learned earlier in this module can represent 28 = 256 different numbers.

18. IP packets Working out the binary equivalent of the IP addresses:
To tackle this problem, we need to take each of the values between the dots one at a time.
Before doing that, it is useful to draw a simple table showing the possible value of each bit in an 8-bit binary number (octet).
If we now add up all of the individual values, we end up with a single denary value for the octet:
= 255.

19. IP packets Another example is given for the octet 01001110:
Adding the new denary values: = 78.

20. IP packets
To convert from decimal to binary we compare with descending powers of two and subtract.
An example shows how to convert the decimal number to binary.

21. IP packets
Working out the binary equivalent of the IP addresses:
To find the binary equivalent for an IP address such as: , we take each of the values between the dots one at a time and find its binary equivalent.
Hence, the binary equivalent for the IP address can be written as four bytes:

22. IP packets
Question: Given that an IPv4 address is a sequence of four one-byte numbers, how many different IP addresses can there be?
Answer: An IPv4 address has 4 bytes each of 8 bits, so there are 4 × 8 = 32 bits.
The total possible combinations of 32 bits = 232 =
IPv6 addresses are a newer version which have been introduced to cope with the increase in demand for addresses.
IPv6 address is 16 bytes long, how many bits is this?
Question: How many different IP addresses does a 16-byte IPv6 address allow?
Answer: An IPv6 has 16 bytes, so there are 16 × 8 =128 bits, so the answer is 2128 = 3.4 × 1038.

23. TCP and connections
A virtual circuit is a link or connection between end devices that enables the two to appear to communicate directly.
On the internet, TCP establishes a virtual circuit between two computers; this connection can then be used to transmit data between the computers.
Suppose you want to look at a web page on the internet, and you type its URL into your browser.
Your computer then makes contact with the server that hosts the web page, requesting to connect to it.
The server sends back a message accepting the request.
Your computer acknowledges this acceptance, after which a connection is established, as shown in Figure 4.

24. TCP and connections This process is called a three-way handshake.
Figure 4
This process is called a three-way handshake.
Once it is complete, the connection is established.
Although nothing has been physically joined, each computer knows that some communication will follow, and is ready for normal data transfer to begin.

25. TCP and connections Data can then be passed between the computers.
This is a two-way process, because when one computer receives data it sends an acknowledgement back to the other (Figure 5).
Figure 5

26. TCP and connections
The server sends the web page data in a sequence of packets.
If a packet goes missing, your computer will not be able to acknowledge the data in that packet.
If the server does not receive an acknowledgement, it will send the packet again.
Figure 6 shows a case with missing packets.
Figure 6

27. TCP and connections
Once the web page has been received correctly, the data transmission is complete.
The connection is no longer needed, so the two computers agree to close it down (Figure 7).
Figure 7

28. Internet essentials To
Figure 8: The postal service analogy of the internet

29. Protocols, layers and stacks
The idea of protocols using the services of other protocols can be illustrated with a visual representation.
We describe this as a four-layer protocol stack (Figure 9).
TCP is in the transport layer and IP is just below it in the internet layer.
TCP establishes a virtual circuit between computers and enables packets of data to be sent between them.
TCP establishes a virtual circuit between computers and enables packets of data to be sent between them.
This relies on IP to route the packets across the various physical links of the internet.
There are further tasks that need to be done to send the data over the physical transmission medium; this is the job of the layer below the internet layer, the network access layer.
So we have a stack of layered protocols.
Figure 9

30. Protocols, layers and stacks
You can see from the figure that one example of a protocol in the application layer, the layer above the transport layer, is the hypertext transfer protocol (HTTP).
This is the standard format for requests and information used by the web.
So, for an example of using a web browser to access Wikipedia:
your browser program creates a request in HTTP format to access data from wikipedia.org
HTTP calls upon the services of TCP to set up a connection to the computer hosting Wikipedia.
TCP uses the services of IP to get the packets of data between your computer and the Wikipedia computer.
IP sets up the route and delivers the packets from node to node between source and destination
the links between the nodes contain the physical transmission medium.

31. Protocols, layers and stacks
Question: What types of physical media might be used to transmit the data?
Answer: Optical fiber, electrical wires or a wireless signal.
Question: The following statements apply to either TCP or IP. Select the correct option in each case (either IP or TCP).
It sets up connections in order to send data between two computers.
It operates in the internet layer.
It addresses packets and routes them across networks.
It operates in the transport layer.
It deals with missing packets.
TCP IP IP
TCP
TCP

32. Protocols, layers and stacks
Now that we have been introduced to protocols at each of the four layers of the TCP/IP protocol stack, we will consider how encapsulation works in practice for a simple connection as shown in Figure 10.
Figure 10

33. Protocols, layers and stacks
User data (a request for a web page) is first given a HTTP header at the application layer.
This header will enable the packet to be read by the web server at the destination.
This packet is then encapsulated and given a TCP header (transport layer) which includes a port number.
A port number is used by TCP to identify all of the packets associated with a particular communication session.
It is in this way that the TCP header ensures that this packet, and any subsequent ones that are part of the same session, are delivered because all packets associated with this web transaction will have the same port number.
Another header (internet layer) is now added, containing the IP address of the destination, and another header (network access layer) is added, containing the Ethernet address of the gateway router.

34. Protocols, layers and stacks
Question:
Match each of the following sentences to the most appropriate layer of the protocol stack.
Used to route packets across the internet.
Used to route packets within the local network.
Delivers all the packets associated with a session.
Looks after the application.
Internet layer
Network layer
Transport layer
Application layer