SQA Higher Computing Unit 3b Computer Networking

Transcription

1 SCHOLAR Study Guide SQA Higher Computing Unit 3b Computer Networking David Bethune Heriot-Watt University Andy Cochrane Heriot-Watt University Tom Kelly Heriot-Watt University Ian King Heriot-Watt University Richard Scott Heriot-Watt University Heriot-Watt University Edinburgh EH14 4AS, United Kingdom.

2 First published 2004 by Interactive University This edition published in 2007 by Heriot-Watt University Copyright c 2007 Heriot-Watt University Members of the SCHOLAR Forum may reproduce this publication in whole or in part for educational purposes within their establishment providing that no profit accrues at any stage, Any other use of the materials is governed by the general copyright statement that follows. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, without written permission from the publisher. Heriot-Watt University accepts no responsibility or liability whatsoever with regard to the information contained in this study guide. SCHOLAR Study Guide Unit 3b: Computing 1. Computing Printed and bound in Great Britain by Graphic and Printing Services, Heriot-Watt University, Edinburgh.

3 Acknowledgements Thanks are due to the members of Heriot-Watt University s SCHOLAR team who planned and created these materials, and to the many colleagues who reviewed the content. We would like to acknowledge the assistance of the education authorities, colleges, teachers and students who contributed to the SCHOLAR programme and who evaluated these materials. Grateful acknowledgement is made for permission to use the following material in the SCHOLAR programme: The Scottish Qualifications Authority for permission to use Past Papers assessments. The Scottish Executive for financial support. All brand names, product names, logos and related devices are used for identification purposes only and are trademarks, registered trademarks or service marks of their respective holders.

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5 i Contents 1 Common Network Protocols What is a network protocol? Telnet HTTP FTP SMTP and POP Lower Layer Network Protocols Summary End of Topic Test The OSI Network Model Introduction to Network Architecture The Application Layer The Presentation Layer The Session Layer The Transport Layer The Network Layer The Data Link Layer The Physical Layer Summary End of Topic Test IP addresses and the Domain Name Service Introduction to IP Addresses and the Domain Name Service The structure of an IP address Classes of IP address Domain Names Limitations of the IP address system Summary End of Topic Test The World Wide Web (WWW) Introduction to the World Wide Web The Structure of a Web Page Types of Browser Search Engines Summary End of Topic Test The Implications of the WWW 43

6 ii CONTENTS 5.1 Introduction to the Implications of the E-commerce Social Implications of the Ethical Implications of the Internet Regulation Summary End of Topic Test Network Security Introduction to Network Security Threats to Network Security Protecting the Network Software and resources Internet Filtering Summary End of Topic Test Denial of Service Attacks and Disaster Recovery Introduction Denial of Service (DoS)Attacks Avoiding Disaster Summary End of Topic Test Data Transmission Introduction Asynchronous and Synchronous Data Transmission Circuit and Packet Switching Unicast, Broadcast and Multicast transmission Ethernet Error Detection and Correction Types of Internet Connection Summary End of Topic Test Wireless Data Applications Introduction Wireless Personal Area Network (WPAN) Wireless Local Area Network (WLAN) Wireless Wide Area Network (WWAN) Summary End of Topic Test Glossary 99 Answers to questions and activities Common Network Protocols The OSI Network Model IP addresses and the Domain Name Service The World Wide Web (WWW) The Implications of the Network Security

7 CONTENTS iii 7 Denial of Service Attacks and Disaster Recovery Data Transmission Wireless Data Applications

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9 1 Topic 1 Common Network Protocols Contents 1.1 What is a network protocol? Telnet HTTP FTP SMTP and POP protocols Setting up servers and clients The structure of an address: Lower Layer Network Protocols Introduction to TCP/IP TCP IP Ports Summary End of Topic Test Prerequisite knowledge Before studying this topic you should know that many programs which are designed to work across a network come in two parts: - a client and a server. The server provides resources to the client and the client uses the resources provided by the server. You should know that on a network, every computer needs a unique address - this is necessary so that information can be correctly routed from one machine to another. On the Internet this unique address is known as the Internet Protocol (IP) address. IP addresses are translated into user-friendly domain names by the Domain Name Service (DNS). You should know that computers use agreed standards in order to make communication easy, and that a common data standard used to transfer information between computers is the American Standard Code for Information Interchange (ASCII). Learning Objectives After studying this topic you should be able to: Understand the need for protocols when connecting computers together in networks

10 2 TOPIC 1. COMMON NETWORK PROTOCOLS Describe the purpose of a number of common network protocols Understand how these protocols are linked to network applications Describe the functions of the networking protocols underlying the Internet

11 1.1. WHAT IS A NETWORK PROTOCOL? 3 Revision Q1: What is an IP address? a) The physical location of a computer b) The code which identifies the user on a network c) A unique number which identifies a computer on a network d) An internet web site Q2: What kind of data is ASCII code used to transmit? a) Text b) Graphics c) Audio d) Binary code Q3: Why do we need a Domain Name Service? a) Because people cannot always remember names of web sites b) To translate user-friendly domain names into IP addresses c) Because not all computers on the Internet have an IP address d) To give every computer on the Internet a unique IP address 1.1 What is a network protocol? A protocol is an agreed set of rules that two parties agree upon in order to communicate successfully. Protocols are the international agreements that make communication possible. In the context of a computer network, a protocol is a set of rules governing things like data format, handshaking (who speaks first) and control commands, which allow computers to communicate. As soon as it was possible to connect two computers together, it became necessary to develop software to enable computers and their users to communicate. The four main application areas where computer communications were developed are: accessing computers remotely accessing information on remote computers

12 4 TOPIC 1. COMMON NETWORK PROTOCOLS transferring files between computers sending and receiving . The protocols associated with these applications are: Telnet File Transfer protocol (FTP) Hypertext Transfer protocol (HTTP) Simple Mail Transfer Protocol (SMTP) and Post Office Protocol (POP3) As we will see later, networks are best understood as a series of software layers with network applications functioning on the top layer. On this top layer, all of these protocols use ASCII code to initiate communication and transfer commands. The applications programs which make use of these protocols have changed over the years to reflect the increased use of Graphical User Interfaces (GUI)s. It is important to remember however that underneath the GUI is a network communications protocol consisting of simple ASCII text commands. All of these protocols rely on the concept of addressability between computers on a network This means that every computer on the network must have a unique identity known as an Internet Protocol (IP) address. 1.2 Telnet One of the first network applications programs developed was one which would allow a user to log into a remote machine. This program allowed them to log on using the machine they were sitting at as if it were a terminal connected directly to the remote machine. The software developed was named Telnet. and had no security built into it whatsoever other than the initial request for a login name and password on the remote system. Telnet has a command driven interface and all communication between the two machines is transferred as ASCII code. A typical Telnet session is started from a Telnet client by issuing a command to access the machine you wish to control. This is normally done by identifying it by its IP address or its domain name plus a port number (A port is a way of identifying a communication channel, allowing several channels to be open simultaneously) The remote machine must be running a Telnet server.

13 1.3. HTTP 5 Telnet is still used as a means of monitoring and controlling network devices such as a hub a switch or a Print Server etc. It can also be used as a primitive diagnostic tool to test web servers, mail servers, news servers and other remote network services. When a telnet connection is made between client and server, the connection is retained until the client disconnects. 1.3 HTTP The Hypertext Transfer Protocol (HTTP) is the protocol which provides many of the functions of the World Wide Web (WWW) today. Tim Berners-Lee is credited with the invention of the WWW He was working at the CERN research facility in Switzerland and had become very frustrated by the problems incurred when trying to access different files on a network of computers which were often running different operating systems and had complex storage systems. He came up with the idea of documents linked to other documents by means of a hyperlink. You can find out more about Tim Berners-Lee from: For the WWW to function, a web server needs to be running on a host machine and a web browser needs to be running on the client machine. (Although of course, server and client can be running on the same physical machine) When you enter a web address, the client machine issues a command to the web server requesting that the server sends the page to the client machine. The protocol used to access pages on the WWW is

14 6 TOPIC 1. COMMON NETWORK PROTOCOLS the Hypertext Transfer Protocol (HTTP) Although essentially this is just a file transfer system, the WWW makes accessing information much easier as the files are transferred transparently without each one being requested separately by the user and in fact the user need not even know where the files are being transferred from, or even if they are all coming from the same computer. Pages stored on the Web server are written in Hypertext Markup Language (HTML) HTML describes a document using embedded formatting controls ( tags) An HTML document is written in ASCII text, so it can be read by any machine which accesses it. When it receives the page, the browser interprets the HTML in order to lay out the text on the screen, requesting additional files such as graphics, multimedia content etc. from the web server as it needs them. HTTP transfers one file at a time and only maintains the connection between client and server while the file is being transferred. You can find out more about the commands available in the HTTP protocol from: Pages on the WWW are identified by a Uniform Resource Locator (URL) A URL contains the protocol used, the server domain name or IP address, and the path-name of the file. e.g. Receiving a web page using Telnet In order to complete this exercise you will need to know the address of your network web server Start up Telnet and connect to your web server on port 80 Enter the following command: GET /index.htm HTTP/1.0 The server will return the contents of the page and close the connection 1.4 FTP File transfer is the moving of a file from one computer to another over a network. The protocol designed to achieve this is the File Transfer protocol (FTP) and it is one of the oldest protocols on the Internet. An FTP server program must be running on a host machine before a client FTP program can access it in order to transfer files to or from that machine. FTP requires an ID and password before the user can transfer files. Although the protocol is an old one, FTP programs have developed from the original command line versions to modern ones which offer an easy to use Graphical User

15 1.4. FTP 7 Interface (GUI) to users, with features such as multiple file selection, batch file transfer, automatic updates of web-sites, and synchronisation options. FTP is still the preferred option for transferring files from a local host to a remote computer when for example a web-site is being created or updated. In a situation like this, the user will have an ID and password which is required to access the remote FTP server, although FTP is not regarded as a particularly secure protocol since the user ID and password are transmitted without encryption. FTP is also the preferred option for downloading files from a remote site for such tasks as software installation or driver upgrades. In a situation like this the user will normally be able to use anonymous FTP where the user ID is "anonymous" and the password is their address. Like Telnet, When an FTP connection is made between client and server, the connection is retained until the client disconnects. Using FTP to receive a file To complete this exercise your tutor will need to have set up an FTP server with anonymous access on your school or college network. Your tutor will tell you the name of the server, the path and the name of the file you should receive. You can see a summary of the FTP command set if you look at the following link: Start up a command window and enter the command: FTP Once the FTP program has started, type in the following instructions: (The response you should get is shown in bold: Open server Connected to server 220 server Microsoft FTP Service (Version 4.0) user (server:( none)):anonymous 331 Annonymous access allowed, send identity ( name) as password Password: username@domain name 230 Anonymous user logged in. cd path 250 CWD command successful. Get filename 200 PORT command successful. 150 Opening ASCII mode data connection for filename (20480 bytes). 226 transfer complete bytes received in 0.17 seconds ( Kbytes/sec) quit

16 8 TOPIC 1. COMMON NETWORK PROTOCOLS 1.5 SMTP and POP protocols Electronic mail allows a message to be composed and sent to another person on a remote system. The protocols associated with electronic mail are the Post Office Protocol (POP3) and the Simple Mail Transfer Protocol (SMTP). SMTP specifies how mail is delivered from one computer to another. This protocol was originally designed for systems which were permanently connected together and allowed to be relayed between machines so that it could be transferred from the sender s mail server across the world to the recipient s mail server. Although SMTP originally did not require any authentication (a mail server would accept mail forwarded from any other client or mail server) the problem of "Spam" (unsolicited commercial ), has meant that most mail servers are set up so that they require authentication before they will accept mail from another machine. The POP3 protocol was designed to allow users to collect mail stored on a remote mailbox. This is useful for users who connect over a dial-up connection and are not permanently connected to their mail server. The POP3 protocol requires the user to provide a username and password, but like Telnet and FTP, these are transmitted in plain unencrypted ASCII code Setting up servers and clients When a you are setting up a mail client such as Eudora or Outlook Express, both the SMTP server and the POP3 server need to be specified. The SMTP server is used for sending mail, the POP3 server for receiving it. POP3 mail servers can also be set up on LANs for delivering within an organisation, and this can be integrated with a POP3 mailbox provided by an Internet Service Provider (ISP) so that internal mail is circulated within the organisation but external is dealt with by the ISP mail server. Ideally this integration is transparent to the user, so that there should be no obvious difference between sending an to a colleague in the next room or one on the other side of the world other than the time it takes for it to arrive at its destination.

17 1.5. SMTP AND POP The structure of an address: An address has the following structure: username@domain name username is the recipient of the , the domain name will usually consist of several words separated by full stops. You will learn more about domain names in the section on IP addressing and domain names. Theoretically the domain name identifies the computer that the account is stored on. The last item in the domain name is called the top level domain and will consist of 2 or 3 letters. The top level domain often gives some indication of your affiliation or country of residence. For example: emile renoir@bigcompany.fr jimslate@smallcompany.co.uk Two letter top level domains are allocated to countries other than the United States. Examples are: Top level Domain.uk.fr.di.au.jp Country United Kingdom France Germany Australia Japan If you get from someone, and there is no "at" sign (@), then that probably means they have the exact same domain as you. For example, if andy@companyname.com sends to barry@companyname.com, Barry might see only Andy in the return address field. Receiving from a POP3 server using Telnet To complete this exercise you will need to know the address of your POP3 mail server, your account name and password. Warning: When using TELNET, your password will be sent in plain ASCII code and if you have set the local echo on, it will be displayed on screen. Start up Telnet on your computer. You want to connect to your POP3 mail server on port 110. The machine you connect to should respond with an appropriate message. Once connected you can retrieve an message in your mailbox using the following commands: user username The mail server should respond with something like: +OK username recipient ok pass password The mail server should respond with something like: +OK username s mailbox has 1

18 10 TOPIC 1. COMMON NETWORK PROTOCOLS total messages (813 octets) Here are some other POP3 commands to try list This command lists the messages in the mailbox retr 1 This command will retrieve the message you have requested - in this case message 1. topn5this command prints the header plus 5 lines of message n dele 1 This command deletes message 1 You can get more information on the POP3 commands by clicking on this link: Extension: As an extension exercise you could try sending an message using the SMTP protocol on port 25 You will need to have a close look at headers and SMTP commands. This is an interesting exercise as it becomes clear how easy it is to send an without having to identify yourself as the sender. You can find out more about the SMTP commands from the following link: You could try posting a message to a news group on your local intranet news server if there is one set up on your network The newsgroup protocol is the Network News Transfer Protocol (NNTP) and the port is 119 You can find out more about the NNTP commands from the following link: Lower Layer Network Protocols Introduction to TCP/IP As you will learn when you study the OSI Network model, computer network functions are divided into different layers to make them easier to implement. Protocols like HTTP and FTP operate at the top layer, but there are other protocols such as TCP and IP which operate on layers lower down. TCP/IP is the acronym for Transmission Control Protocol / Internet Protocol. These are the two protocols which have made connecting networks together and connecting to the Internet easy to achieve. TCP/IP was developed by the US Defence Advanced Research Project Agency (DARPA). DARPA implemented a Wide Area Network called ARPANET in the late 1960s which connected mainframe computers together over long distance telephone lines. ARPANET has since expanded beyond any of the expectations of its inventors and has grown into the Internet that we know today. TCP/IP has been adopted internationally as the standard for connecting computers to the Internet.

19 1.6. LOWER LAYER NETWORK PROTOCOLS 11 TCP/IP is actually made up of two protocols. The Transmission Control Protocol (TCP) which provides a reliable end-to-end link between hosts, and the Internet Protocol (IP) which defines a common method for exchanging packets among a wide collection of networks TCP When a file is to be transmitted between two computers on a network running TCP/IP then the file is broken down into chunks of data called packets. TCP is responsible for splitting the data into these packets and adding a header to each one. This header includes a sequence number which allows the file to be recreated at the receiving end by re-assembling the packets in the correct order. TCP is also responsible for retransmitting packets if it does not receive an acknowledgment that a packet has been received. On the same layer as TCP, the User Datagram Protocol (UDP) provides an unreliable Datagram delivery channel - UDP does not request re-transmission of lost packets and is used for services such as audio or video streaming where it is not so important that every packet is received IP The IP protocol is responsible for taking each datagram packet and adding its own header to allow the packet to be routed around between networks. As it is perfectly possible for packets to follow different routes to their destination, they might easily take different amounts of time to arrive and therefore be out of sequence when they do. As long as the packets can be re-assembled in the correct order by TCP using the sequence numbers which were added when the packets were created, then the transmission will have been successful and TCP will issue an acknowledgment.

20 12 TOPIC 1. COMMON NETWORK PROTOCOLS Ports TCP and UDP introduce the concept of a port, since we often need several network connections to be functioning at the same time. Some common ports and the services that run on them are: Service Port File Transfer Protocol (FTP) 21 and 20 Telnet 23 Simple Mail Transfer Protocol (SMTP) 25 Hyper Text Transfer Protocol (HTTP) 80 Post Office Protocol (POP3) 110 By specifying ports and including port numbers with TCP/UDP data, multiple network connections can take place simultaneously. This is known as multiplexing, and is why you can collect your and browse the Internet at the same time. The port number, along with the source and destination addresses for the data, together make a socket. 1.7 Summary Telnet is a protocol used to connect to a remote computer FTP is a protocol used to transfer files to and from a remote computer HTTP is a protocol used to retrieve web pages and other files from a web server SMTP is a protocol used to transfer to a mail server POP3 is a protocol used to retrieve from a mailbox on a mail server An address consists of a username and a domain name separated by symbol Telnet. FTP, HTTP, SMTP and POP3 all use plain text ASCII commands, although nowadays GUI front ends exist to make them easier to use TCP is a lower level protocol responsible for splitting a file to be transmitted into packets, each with a sequence number IP is a lower layer protocol responsible for routing packets around between networks A combination of a port and a TCP/IP connection is called a socket and allows several virtual connections to run on one machine at the same time. 1.8 End of Topic Test An online assessment is provided to help you review this topic.

21 13 Topic 2 The OSI Network Model Contents 2.1 Introduction to Network Architecture The Application Layer The Presentation Layer The Session Layer The Transport Layer The Network Layer The Data Link Layer The Physical Layer Summary End of Topic Test Prerequisite knowledge Before studying this topic you should know the difference between a node and a channel on a network. You should be familiar with the functions of a hub (multiport repeater), a switch and a routeron a network. You should be familiar with the idea of a network protocol, and the fact that different protocols operate at different levels - for instance the SMTP protocol operates at the level of an application such as an client, whereas the TCP/IP protocol operates at the level of a node on the network. A host on a network usually means a node which provides a service - in effect a machine which can act as a server.

22 14 TOPIC 2. THE OSI NETWORK MODEL Learning Objectives After studying this topic you should be able to: Name and describe the functions of the 7 layers in the OSI Network model Understand the reasons for dividing network functions into a hierarchy of different layers Understand the reasons for Independence and transparency in network layers Revision Q1: What is a node in a network? a) A device on a network with a unique IP address b) A connection between two computers c) An Internet link d) A type of communications software Q2: What is a channel on a network? a) A device on a network with a unique IP address b) A connection between two nodes c) An Internet link d) A type of communications software Q3: What is these statements is false? a) A hub just retransmits packets to every node on the network b) A switch selectively retransmits packets according to the IP address of the destination machine c) A hub cannot reduce the number of collisions on a local area network d) A switch cannot reduce the number of collisions on a local area network

23 2.1. INTRODUCTION TO NETWORK ARCHITECTURE Introduction to Network Architecture The Open Systems Interconnection (OSI) model was developed to ensure that communications equipment and networking software would be compatible. The OSI model divides the business of data transfer across a network into a number of different layers. The layers are hierarchical with each layer having a specific task to perform. The top layer is the applications layer which is the layer you are using when you send an using an client or view a web page using a web browser. The lowest layer is the physical layer which is concerned with the cabling and physical characteristics of the network. Each layer has been designed with a particular task in mind and communicates with the layers above and below it in the hierarchy. Each layer is independent. The OSI model is often referred to as the OSI network architecture There are several good reasons for using this hierarchical layered model: Transparency: From the point of view of each layer on a network, it behaves as if it is communicating directly with the equivalent layer on the receiving network. This means that from the user s point of view, the application layer on one network appears to pass information to the application layer on the other. In effect each layer actually passes data to the layer below when sending information and to the layer above when receiving information. In reality of course it is the physical layer (the cabling) which actually transmits the data. Hierarchy: From the point of view of the Application layer, the network application is transferring files such as messages, images or web pages to the equivalent application on the computer it is connected to. As the data is passed by one layer to the layers below, these files are split into smaller segments, each with that layer s headers, error correction and sequence information added to them, until the data becomes a stream of bits transmitted via the physical layer. The reverse process takes place at the receiving end with the headers getting stripped out, segments reassembled until the application layer presents the user with the transmitted file. Each layer is responsible to the layer above and below it for ensuring that the data it receives and transmits remains intact. Independence: Because the layers are independent, it is possible to replace or upgrade one layer with another software version without having to change how the other layers communicate with it. This modularity means that parts of the system can be upgraded independently without changing the entire system. If you want to read an analogy which describes the functions of the 7 OSI layers look at this link The Application Layer The Application Layer is the top layer of the OSI model. It provides a set of interfaces for applications to obtain access to networked services as well as access to the kinds of network services that support applications directly such as an client, or an Internet browser.

24 16 TOPIC 2. THE OSI NETWORK MODEL Application Layer protocols include HTTP, FTP, SMTP and POP The Presentation Layer The Presentation Layer handles data format information for networked communications. This is done by converting data into a standardised format that can be understood by both sides. The presentation layer might also be responsible for character code conversion, compression or encryption. It also controls file locking and security at the user level - this is why if a file is already open for writing on a network drive, another user cannot also open it for writing. Standards such as MIDI, HTML, GIF, ASCII might be used at this layer. 2.4 The Session Layer The Session layer manages log on procedures and password recognition. It permits two parties to hold ongoing communications called a session across a network. 2.5 The Transport Layer The Transport layer breaks up the file into segments for transport to the network, and combines incoming segments into a contiguous file. The Transport layer is responsible for creating and maintaining the basic end-to-end connection between communicating systems, ensuring that the bits delivered to the receiver are the same as the bits transmitted by the sender; in the same order and without modification, loss or duplication. The transport layer guarantees an error free connection between two hosts. The TCP and UDP protocols operate at this level. Typically the transport layer and those above it run on hosts in a network and not on intermediate equipment in between whereas the lower three layers are implemented on all network nodes such as routers switches or hubs. 2.6 The Network Layer The Network layer is concerned with the path through the network. It is responsible for routing, and controlling the flow of information between hosts. The Network layer converts the segments into smaller datagrams that the network can handle: network hardware source and destination addresses are also added. The Network layer does not guarantee that the datagram will reach its destination. The Network layer works in units of packets. The IP protocol is used at this layer.

25 2.7. THE DATA LINK LAYER 17 Network Routers operate at this level as they are responsible for routing packets between networks. 2.7 The Data Link Layer The data link layer is responsible for putting data into frames or packets along with error checking data. In an Ethernet network the Data Link layer works in units of frames. In the opposite direction the Data Link Layer transforms a stream of raw bits (0s and 1s) from the physical into a data frame and provides an error-free transfer from one node to another, allowing the layers above it to assume virtually error-free transmission. The Data Link layer is a firmware layer of the Network Interface Card. (NIC) where the Ethernet network standard is implemented. Network switches also operate at this layer. 2.8 The Physical Layer The function of the Physical Layer is to transform bits in a computer system into electromagnetic (or equivalent) signals for a particular transmission medium (wire, fibre, ether, etc.) The Physical Layer defines items like the type of cabling (coax, twisted pair, etc.), the frequency of operation (10 Mbps, 100Mbps, etc.), voltage levels, and network topology (star, bus, ring, etc) The Physical layer functions in units of bits. Network hubs and repeaters operate at this layer. Try these links to find out more about the OSI/ISO architecture model ppin.pdf Although for obvious reasons, electronic communication is used in computer networks, the protocols themselves do not actually stipulate this. There exists an implementation of TP/IP using pigeons as carriers, which although suffering from low bandwidth, illustrates the fact that the protocols are not specific to any one communications medium. You can read more about this idea here: Summary The OSI model divides networks into seven layers Layers are hierarchical, transparent, and independent

26 18 TOPIC 2. THE OSI NETWORK MODEL The Application layer provides interfaces for network applications The Presentation layer handles data format information The Session layer manages log-on and password authentication The Transport layer breaks up file into segment for transport over a network and guarantees that these segments are not lost The Network layer routes packets The Data Link layer guarantees error free transmission The Physical layer transmits bits over physical medium 2.10 End of Topic Test An online assessment is provided to help you review this topic.

27 19 Topic 3 IP addresses and the Domain Name Service Contents 3.1 Introduction to IP Addresses and the Domain Name Service The structure of an IP address Classes of IP address Private IP addresses Subnetmask Localhost Static and Dynamic IP addressing Domain Names Domain Name Service (DNS) Limitations of the IP address system Summary End of Topic Test Prerequisite knowledge Before studying this topic you should know that computers on the Internet are uniquely identified by their IP address and that the World Wide Web (WWW) uses a Name Resolution system so that user friendly domain names can be mapped to a machine s IP address. You should know that computers on a network which provide a service to other users are often referred to as hosts. Learning Objectives After studying this topic you should be able to: Describe the structure of an IP address Explain the three different classes of IP address Understand the difference between static and dynamic IP addressing Understand some of the limitations of IP addressing Describe the Domain Name Service (DNS)

28 20 TOPIC 3. IP ADDRESSES AND THE DOMAIN NAME SERVICE Revision Q1: Which of these is a host? a) A network link b) A networked printer c) A networked machine running a mail server d) A multi-port repeater (hub) Q2: Which of the these is not a host? a) A networked machine running a web server b) A print server c) A router d) A multi-port repeater (hub) Q3: What is an IP address? a) The physical location of a computer b) The code which identifies the user on a network c) A unique number which identifies a computer on a network d) An internet web site Q4: Why do we need a Domain Name Service? a) Because people cannot always remember names of web sites b) To translate user-friendly domain names into IP addresses c) Because not all computers on the Internet have an IP address d) To give every computer on the Internet a unique IP address 3.1 Introduction to IP Addresses and the Domain Name Service An Internet Protocol (IP) address is the way the Internet Protocol distinguishes computers (or more specifically Network Interfaces) that exist on the same network. On any one network, you cannot have two computers sharing an IP address, as this would make it impossible to distinguish between them when trying to send data from one computer to another. You can however have a single computer with two network Interface Cards (NIC)s fitted, each one with its own IP address. In this case the computer would be connected to two different networks. Routers are computers like this. All computers that are connected together on the Internet (or on a private LAN) need unique IP addresses. Because IP addresses are difficult to remember, we use Domain names to identify them instead. When we connect to a remote computer using a domain name such as google.co.uk, a system known as the Domain Name Service (DNS) translates the domain name into an IP address ( in this case) so that the correct computer can be found.

29 3.2. THE STRUCTURE OF AN IP ADDRESS The structure of an IP address An IP address is a 32-bit number separated in to four 8-bit parts called octets. The four parts like a postal address, with the difference that the detail is ordered from right to left instead of left to right. The first number of the IP address is the most general and the last is the most specific. An IP address can be divided into 2 sections, - the network identifier and the host identifier. There are three ways to split the IP address. Class A Class B Class C nnn.hhh.hhh.hhh nnn.nnn.hhh.hhh nnn.nnn.nnn.hhh where n = network identifier, h = host identifier The network identifier is used to route packets to the correct destination network. Once a packet reaches the network which requested it, the network part of the address is ignored and the packet is routed to the correct host using the host identifier. 3.3 Classes of IP address Since each computer on the Internet needs a different IP address, there has to be some way of sharing out the IP addresses so that large companies and organizations have one for each of their computers, while letting smaller organizations have some as well. Since there are a small number of large organizations and a large number of small organizations, ranges of IP addresses can be allocated by giving large organisations blocks of Class A addresses and small organisations blocks of Class B or Class C addresses. Class A addresses have their first octet in the range 1 to 126. Class B addresses have their first octet in the range 128 to 191 and Class C addresses have their first octet in the range 192 to 223. A very large company with very complex internal networks may be allocated a Class A address block such as 115.*.*.*. This would give a total of about 16 million possible IP addresses. There are only 127 Class A address blocks, and no more are to be allocated. Class B addresses are common for large companies, allowing a block of around IP addresses. When a Class B IP address is allocated, (say *.*), the first two numbers identify that company network. The company can give any IP address in that block to any computer on their network. Class C addresses are the third type, giving 254 possible IP addresses for any one block (0 and 255 are reserved for particular functions). Here, the first three octets are specified, and the remaining field is allocated by the owner of the address. Networks that are directly connected to the Internet are usually connected to an Internet Service Provider (ISP) via a full time connection (such as a cable or leased line) The ISP informs the network administrator which IP addresses can be used on the network, and a router controls the delivery of packets to the appropriate IP address on

30 22 TOPIC 3. IP ADDRESSES AND THE DOMAIN NAME SERVICE the network. Small ISPs typically have 1 or 2 Class C licenses, providing 250 to 500 IP addresses. When you dial up an ISP with a modem, your computer is temporarily allocated an IP address. This will be in the range of the Class C licences that they own. If you want to know more about classes of IP address have a look at the following links: There are two other special classes of IP addresses: Class D addresses have their first octet in the range 224 to 239 and are used for multicast. Multicast is a way of defining a group of nodes and only sending packets to these nodes instead of every node on the network (Broadcast) Class E addresses have their first octet in the range 240 to 255 and these are reserved for experimental purposes.

31 3.3. CLASSES OF IP ADDRESS Private IP addresses Private IP addresses are ranges of IP addresses that are "known not to exist " on the Internet. This means that no computer on the Internet will ever be assigned these addresses. These can safely be used in internal Local Area Networks (LAN)s, as they have no direct connection to the Internet. One example of a Private IP range is from to The private IP ranges that are not allocated on the Internet are: to Class A to Class B to Class C Note that 0 and 255 are reserved in any class is the global broadcast address. The advantages to using private IP addresses on a network is that you can use a proxy server with single public IP address to access the network. The proxy server forwards requests for web pages or other Internet services as if they were its own. This means that only one IP address is presented to the outside world, protecting machines on the network from external attack, and avoiding the purchase of more than one IP address. This arrangement is known as Network Address Translation (NAT)

32 24 TOPIC 3. IP ADDRESSES AND THE DOMAIN NAME SERVICE Subnetmask A local Area Network (LAN) might uses the private IP range to When assigning an IP address to a machine a subnetmask must be specified. In the case of a small network, the subnetmask could be used. A subnetmask is a way of saying which bits to ignore in the address when checking for a match. So if the subnetmask is then it ignores the last 8 bits of the address (last octet) when checking to see if an IP address applies to the destination network or not.

33 3.4. DOMAIN NAMES 25 When matching an IP address, a network machine will try the following options in this sequence: 1 Look for an exact match (equivalent to a subnetmask of ) Look for a match with the existing subnetmask ( ) i.e. an address in 2 the range to in this case. 3 Use the default route (usually the IP address of the router) This means that a local network address will always be matched to the correct machine before checking for that address on the Internet Localhost Localhost is a special term in TCP/IP is the localhost (loopback interface) this is a software only interface, which is internal to the machine, and is not accessible over any external connection. It does not matter what the IP address of your machine is, will always refer to the local computer. This means that this interface can only be accessed from the computer itself. If a computer wants to talk to itself on a different port, it can use localhost or If you have a web server running on a machine, then you can test out internet pages on that server without a network connection by connecting to the address: The command ping localhost will return the IP address as well as the name of your computer Static and Dynamic IP addressing Static IP addressing is where every host on a network has a fixed IP address. Dynamic IP addressing is where hosts are allocated IP addresses when they are connected to the network, but that IP address can be used by another machine when the host is disconnected. Dynamic IP addressing is often implemented where there are more potential hosts than there are IP addresses available, for instance where an ISP has a range of IP addresses and these are dynamically allocated to customers when they connect to the Internet via a modem. On a LAN, dynamic IP addressing is often used in order to avoid the possibility of two machines being allocated the same IP address. The server runs a DHCP (Dynamic Host Configuration Protocol) service which allocates IP addresses from a range specified by the network manager to machines whenever they connect) 3.4 Domain Names A Domain Name is the unique name that identifies an Internet site. Domain Names always have 2 or more parts, separated by dots. The part on the left is the most specific, and the part on the right is the most general. For example in the domain name: my comany.com

34 26 TOPIC 3. IP ADDRESSES AND THE DOMAIN NAME SERVICE my company refers to a specific company and.com refers to the commercial domain. Examples of three letter top level domains are: Top level Domain Meaning.com Commercial business, a company.net Network provider, Internet Service Provider.gov Governmental agency.edu Educational institution.org Non-profit institution.mil US. military.int International organisation Two letter top level domains are allocated to countries other than the United States. Examples are: Top level Domain.uk.fr.di.au.jp Country United Kingdom France Germany Australia Japan Note that my company.com and my company.co.uk are treated as different domains, but may point to the same Internet site. Two or more domain names may point to the same machine, but a domain name can only ever refer to one machine. Many companies will register their names in a number of top level domains so that people can find them easily, and so that they do not get their company name hijacked by competitors. If you want to know more about top level domains have a look at he Internet Assigned Numbers Authority (IANA) web site It is possible for a Domain Name to exist but not be connected to an actual machine. This is often done so that a group or business can have an Internet address without having to establish a real Internet site. In these cases, some real Internet machine must handle the mail on behalf of the listed Domain Name. Name resolution is the system of mapping a domain name to its IP address. This is done on the Internet by the Domain Name Service. The domain name system on the WWW is regulated by an organisation called InterNIC. If an company wishes to have a presence on the WWW then the first step is usually to pay an accredited registrar a to register one or more domain names for them. In the UK the organisation responsible for overseeing the registration is Nominet. Once that has been done, the next step is to pay to host a web site. The company hosting the web site will provide details of the name servers which will resolve the domain name to an IP address and this information needs to be given to the registrar. Once this information has propagated around the WWW, typing the domain name into a browser will result