An Introduction to Networking

An Introduction to Networking
Chapter 1 Updated January 2007 Panko’s Business Data Networks and Telecommunications, 6th edition Copyright 2007 Prentice-Hall May only be used by adopters of the book Notes: Plain text—things to say to the students < > Meta information or suggestions about how to teach [ ] Extra information not in the text <Note: Beginning on Slide 48, students should have a printed copy of the PDF file for Figure 1-12.> [Some instructors like to begin with a brief blurb on how networking is one of the core competencies required of all IS graduates and how the demand for networking jobs generally is the fastest growing of all IS jobs.] [If you can, bring some examples of networking equipment to class—wiring, switches, routers, NICs, etc.] Copyright 2005 Prentice-Hall

Builds Slides with the “mouse click” icon in the upper right hand corner are “build” slides Not everything on the slide will appear at once Each time the mouse click icon is clicked, more information on the slide will appear. <This is a note primarily for you. However, students should understand builds it if they are working from copies of slides; otherwise, they may be confused when everything doesn’t appear at once.> <Read the slide.> Copyright 2005 Prentice-Hall

Part I: Basic Networks Concepts
Concepts we will see throughout the book Let’s begin with a few concepts that we will see throughout the book. Copyright 2005 Prentice-Hall

Figure 1-1: Basic Networking Concepts
What Is a Network? A network is a transmission system that connects two or more applications running on different computers. <Read the definition and emphasize that networking is about getting applications to talk to one another.> Users only care about applications. The rest is details they don’t care about. If networking people do their jobs well, then users can focus on the applications. It is our job to make networking invisible to the user. Network Copyright 2005 Prentice-Hall

Figure 1-1: Basic Networking Concepts
Client/Server Applications Most Internet applications are client/server applications Clients receive service from servers The client is often a browser Client Program Server Program <This slide continues discussing applications.> <Go through the slide.> [IS majors who see themselves as programmers or database specialists should understand that the programs they write to work with databases and other resources will be client/server programs. They will need to understand networking to get their client/server programs to work together well.] Services Client Computer Server Computer Copyright 2005 Prentice-Hall

Part II: The Nine Elements of a Network
Although the idea of “network” is simple, you must understand the nine elements found in most networks We will now look inside networks to get a better feeling for their operation. Copyright 2005 Prentice-Hall

Figure 1-3: Elements of a Network
Client Application Server Application Message (Frame) Switch 2 Access Line 1. Networks connect applications on different computers. Client Computer Server Computer Switch 1 Switch 3 Networks connect computers: 2. Clients (fixed and mobile) and 3. Servers Trunk Line Again, applications are the key thing to users. Unless two applications can exchange data, networking is useless. <Read the first box on the slide.> <Read the second box. Recap the difference between clients and servers. Note that many clients today are mobile devices.> Mobile Client Outside World Wireless Access Point Router Copyright 2005 Prentice-Hall

Client Application Server Application Message (Frame) Client Computer Server Computer 4. Computers (and routers) usually communicate by sending messages called frames Switch 1 Switch 3 Trunk Line <Read the box.> <Emphasize that messages in single networks are called frames. Later, we will see another type of message—a packet.> Mobile Client Outside World Wireless Access Point Router Copyright 2005 Prentice-Hall

Client Application Server Application Message (Frame) Client Sends Frame to Sw1 Sw2 Sends Frame To Sw3 Sw1 Sends Frame to Sw2 Switch 2 Client Computer Server Computer Sw3 Sends Frame to Server Switch 1 Switch 3 Trunk Line This slide shows how devices called switches move a frame across a network. <Go through the build.> Each switch along the way decides in turn where to send the frame next. Mobile Client 5. Switches Forward Frames Sequentially Switch 4 Outside World Wireless Access Point Router Copyright 2005 Prentice-Hall

Figure 1-5: Ethernet Switch Operation
C3- is out Port 15 D C4-B6-F9 Switching Table Port Host 10 A1-44-D5-1F-AA-4C 13 B2-CD-13-5B-E4-65 15 C3-2D-55-3B-A9-4F 16 D C4-B6-F9 Switch 2 Port 15 3 Frame to C3… Frame to C3… This slide show how Ethernet switches operate. Ethernet is a very popular network technology. <Go through the build to show how each switch forwards frames.> Station A1-… creates a frame for station C3-… 1. Station A1-… send the frame to the switch. 2. The switch notes that Station C3-… is connected to Port 15 on the switch. 3. The switch sends the frame out Port 15, to station C3-… . 1 C3-2D-55-3B-A9-4F A1-44-D5-1F-AA-4C B2-CD-13-5B-E4-65 Switch sends frame to C3- A1- sends a frame to C3- Copyright 2005 Prentice-Hall

Client Application Server Application Message (Frame) Switch 2 Access Line Client Computer Server Computer 6. Wireless Access Points Connect Wireless Stations to Switches Switch 1 Switch 3 Trunk Line Mobile devices communicate with wireless access points via radio. Each wireless access point connects to a switch. The access point relays messages between the mobile clients and the switched network. Mobile Client Switch 4 Outside World Wireless Access Point Router Copyright 2005 Prentice-Hall

Client Application Server Application Message (Frame) Switch 2 Access Line Client Computer Server Computer 7. Routers connect networks to the outside world; Treated just like computers in single networks Switch 1 Switch 3 Trunk Line We saw earlier that switches forward frames within networks. Routers connect the network to the outside world—to other networks <Note that the router is IN the network. It has to be in order to connect the network to the outside world.> Mobile Client Switch 4 Outside World Yes, single networks can contain routers Wireless Access Point Router Copyright 2005 Prentice-Hall

Client Application 8. Access Lines Connect Computers to Switches Server Application Message (Frame) Access Line Switch 2 Client Computer Server Computer Switch 1 Switch 3 Trunk Line The devices in the network are connected by transmission lines. <Read through the build.> Mobile Client 9. Trunk Lines Connect Switches to Switches and Switches to Routers Switch 4 Outside World Wireless Access Point Router Copyright 2005 Prentice-Hall

Figure 1-4: Packet Switching and Multiplexing
Breaking Communications into Small Messages is Called Packet Switching, even if the Messages are Frames AC AC Client Computer A AC Server Computer C AC AC BD AC Trunk Line Multiplexed Packets Share Trunk Lines So Packet Switching Reduces the Cost of Trunk Lines BD Access Line BD BD Breaking communications into small messages is called packet switching, even if the messages are called frames instead of packets. Multiplexing mixes the messages of multiple conversations on a trunk line. <Go over the figure—show AC messages from A to C and BD messages from B to D.> Note that AC and BD messages are mixed on the trunk line between the two switches. Packet switching reduces trunk line costs because conversations share the trunk line’s capacity. This is far cheaper than having a transmission line for each conversation, Just as it is cheaper to have many cars share the lanes in a freeway rather than giving each car its own lane. [Other costs actually are increased; for instance, packet switches are more expensive than other switches. However, trunk lines are so expensive that total costs do fall.] Router D Mobile Client Computer B Copyright 2005 Prentice-Hall

Network Elements: Recap
Name the 9 Elements of Single networks. Without looking back through your handout Never talk about an innovation “reducing cost,” “increasing speed,” etc. without specifying which element is cheaper or faster. For example, multiplexing only reduces the cost of trunk lines; other costs are not decreased <Note: You might have your students turn over the handout and write on the back.> <You might list what they remember on the board.> Applications (the only element that users care about) Messages (Frames) Computers Clients Servers Switches and Routers Transmission Lines Trunk lines Access Lines Wireless Access Points <Read the box on the left.> Copyright 2005 Prentice-Hall

Part III: Transmission Speed
The first question people ask about a new-born baby is, “Is it a boy or a girl?” The first question people ask about a network is, “How fast is it?” Copyright 2005 Prentice-Hall

Figure 1-6: Transmission Speed
Measuring Transmission Speed Measured in bits per second (bps) In metric notation: Increasing factors of 1,000 … Not factors of 1,024 Kilobits per second (kbps)-note the lowercase k Megabits per second (Mbps) Gigabits per second (Gbps) Terabits per second (Tbps) Speed is measured in bits per second. A bit is a single one or a single zero. Note that speeds are expressed in metric notation. Speed designations increase by a factor of 1,000---not 1,024 as in computer memory. Note that the correct metric designation for kilobits per second is kbps with a small k. [In the metric system, Capital K is Kelvins.] [You might remark snidely that networking people know the metric system, while computer people usually do not.] Copyright 2005 Prentice-Hall

Measuring Transmission Speed What is 23,000 bps in metric notation? What is 3,000,000,000 in metric notation? What is 15,100,000 bps in metric notation? Occasionally measured in bytes per second If so, written as Bps Usually seen in file download speeds <This slide gives some exercises in using the metric notation learned on the previous slide.> 23,000 bps is 23 kbps. 3,000,000,000 bps is 3 Gbps. 15,100,000 bps is 15.1 Mbps. Note that speed is sometimes given in bytes per second This may be done for file downloads Byte is represented by capital B, so bytes per second is Bps Copyright 2005 Prentice-Hall

Writing Transmission Speeds in Proper Form The rule for writing speeds (and metric numbers in general) in proper form is that there should be 1 to 3 places before the decimal point 23.72 Mbps is correct (2 places before the decimal point). 2,300 Mbps has four places before the decimal point, so it should be rewritten as 2.3 Gbps (1 place). 0.5 Mbps has zero places to the left of the decimal point. It should be written as 500 kbps (3 places). <Read the slide.> Copyright 2005 Prentice-Hall

Writing Transmission Speeds in Proper Form How to convert 1,200 Mbps to proper form Divide the number 1,200 by 1000 Move decimal point three places to the left: 1.200 Multiply the metric suffix Mbps by 1,000 Gbps Result: 1.2 Gbps In writing speeds, there is a number and a metric suffix. If you divide the number by 1,000 to put in in proper form, move the decimal points three places to the left: 1,200 becomes To compensate, you must multiply the suffix by 1,000—in this case, Mbps to Gbps. When you make either the number or the metric smaller, you have to make the other one bigger. Copyright 2005 Prentice-Hall

Writing Transmission Speeds in Proper Form How to convert Mbps to proper form Multiply the number by 1000 Move decimal point three places to the right: 36 Divide the metric suffix Mbps by 1,000 kbps Result: 36 kbps Again, there always is a number and a metric suffix. If you multiply the number by 1,000, move the decimal points three places to the right: becomes 36. To compensate, you must divide the suffix by 1,000—Mbps to kbps. When you make either the number or metric bigger, you have to make the other one smaller. Copyright 2005 Prentice-Hall

Writing Transmission Speeds in Proper Form How should you write the following in proper form? kbps 0.47 Gbps 11,200 Mbps .0021 Gbps <Give students some time to work out these examples> has three places to the left of the decimal point. It is OK. No change. 0.47 has nothing to the left of the decimal point. Leading zeros don’t count. Multiply the number by 1,000—move the decimal point three places to the right to get 470, and divide the suffix by 1,000 to get Mbps. The answer is 470 Mbps. 11,200 has 5 places before the decimal point Divide 11,200 by 1,000—move the decimal point three digits to the right— to get , and the metric suffix Mbps by 1,000 to get Gbps. The answer is 11.2 Gbps 0.002 has nothing to the left of the decimal point. Leading zeros don’t count. Multiply the number by 1,000—move the decimal point three places to the right--to get 2.1, The answer is 2.1 Mbps. Copyright 2005 Prentice-Hall

Rated Speed The speed in bits per second that you should get (advertised or specified in the standard). Throughput The speed you actually get Almost always lower than the rated speed On Shared Transmission Lines Aggregate throughput—total throughput for all users Individual throughput—what individual users get An important distinction in networking speeds is the difference between rated speed and throughput. <Read the slide.> Copyright 2005 Prentice-Hall

Part IV: LANs and WANs When we talk about networks, there are two types of networks—local area networks (LANs) and Wide Area Networks (WANs). In this section, we will look at the differences between them. Copyright 2005 Prentice-Hall

Figure 1-8: LANs Versus WANs
Characteristics Scope LANs WANs For transmission within a site. Campus, building, and SOHO (Small Office or Home Office) LANs For transmission between sites Campus LAN Building LAN Note that the difference between LANs and WANs is not about distance by itself—it is about whether a network is within a firm’s site (LAN) or between sites (WAN). Wide Area Network Home LAN Copyright 2005 Prentice-Hall

Characteristics LANs WANs Cost per bit Transmitted Low High Typical Speed Unshared 100 Mbps to a gigabit per second to each desktop. Even faster trunk line speeds. Shared 128 kbps to several megabits per second trunk line speeds Note in the second row that long-distance transmission is expensive <Compare the price of a local call to the price of a long-distance or international call.> Note in the third row that as a consequence of cost per bit transmitted, typical speeds are quite different in LANs and WANs. In economics, if something becomes more expensive per unit, then people will buy fewer units. LANs typically bring 100 Mbps to a gigabit per second to each desktop. WANs only have speeds of 128 kbps to several megabits per second—and this is shared. It is critical for students, who traditionally deal with LANs, to understand how different cost and speed are in WANs. It’s simple economics. If the cost per unit is higher, the number of units demanded will be lower. Corporations cannot afford high-speed for most of their WAN transmission Copyright 2005 Prentice-Hall

Characteristics LANs WANs Management On own premises, so firm builds and manages its own LAN or outsources the Work Must use a carrier with rights of way for transmission in public Area. Carrier handles most work but Charges a high price. Choices Unlimited Only those offered by carrier Because LANs are on your own premises, you have to manage them. [As one guru once said, anything you own ends up owning you.] [Of course, for networking students this is good, because it means more jobs.] With WANs, you cannot lay your own wires. [Imagine running wires through your neighbor’s yard!] Carriers are companies to whom the government gives rights of way to lay wire in a city or area. They handle most of the work (albeit at high cost). Because you own the LAN, you can use any technology you wish. However, carriers often only offer a limited number of choices for firms. Copyright 2005 Prentice-Hall

Figure 1-9: Local Area Network (LAN) in a Large Building
Wall Jack Workgroup Switch 2 Client Server Workgroup Switch 1 Wall Jack To WAN Core Switch Router Before we begin to click through the build, let’s note the basic organization of the network: There is a workgroup switch on each floor. It serves the computers on its floor. Wiring runs from workgroup switches to wall jacks for individual computers on the floor. There is a core switch in the basement. Wiring runs from the core switch to each workgroup switch. <Begin clicking through the build to follow a frame sent from the client to the server on another floor Note that All traffic between floors goes through the workgroup switch.> [Could the core switch be eliminated, so that connections would go directly between workgroup switches? The simple answer is, “Yes.” However, this tends to overload workgroup switches in middle floors, which have to pass on traffic between most floors Analogy: Think of sitting in the middle of a long table during Christmas dinner.] Frames from the client to the server go through Workgroup Switch 2, through the Core Switch, through Workgroup Switch 1, and then to the server Copyright 2005 Prentice-Hall

Part V: Internets So far we have been looking at networks.
However, routers can connect the network to the outside world. Routers allow groups of networks to be created. These groups of networks are called internets. Copyright 2005 Prentice-Hall

Figure 1-11: Internets Single LANs Versus Internets
In single networks (LANs and WANs), all devices connect to one another by switches—our focus so far. In contrast, an internet is a group of networks connected by routers so that any application on any host on any single network can communicate with any application on any other host on any other network in the internet. This is an important slide. So far, we have been looking at single networks. <Read first bullet point.> Now we will introduce another major concept, internets. <Read second bullet point.> [Historically, single networks—LANs and WANs—came first, in the 1960s and 1970s. Then, Vint Cerf invented the concept of internetworking in the late 1970s in the to like these single networks together, allowing people to work across networks.] [Cerf originally called internetworks “catenets” based on the computer science term “concatenation.”] Application Application WAN LAN LAN Router Copyright 2005 Prentice-Hall

Host Figure 1-11: Internets Internet Components
All computers in an internet are called hosts Clients as well as servers Client PC (Host) Cellphone VoIP Phone PDA Server Internet ALL computers connected to an internet are called host computers. This is true of servers. <Most students find this obvious.> However, individual office PCs also are host computers if they are attached to an internet. So are PDAs, cellphones, and any other devices attached to an internet. <Question: “Is your home PC a host when it is connected to the Internet? Answer: “Yes.”> <Question: “Is you’re a PC in the school’s lab a host when it is connected to the Internet? Answer: “Yes.”> [Cats are not hosts because they ignore the Internet ;)] Cat (Ignores Internet) Copyright 2005 Prentice-Hall

Figure 1-11: Internets Hosts Have Two Addresses IP Address
This is the host’s official address on its internet 32 bits long Expressed for people in dotted decimal notation (e.g., ) Single-Network Addresses This is the host’s address on its single network Ethernet addresses, for instance, are 48 bits long Expressed in hexadecimal notation (e.g., AF-23-9B-E ) This is an important point and a bit difficult for some students. <Read the slide. Each major bullet is a build.> When the Internet was created, there were many single network technologies with different addressing systems. For delivery to any host on an internet, an additional addressing system was needed. There is no exact analogy outside networking. However, as a student, you have a local ID number (at your university) and probably have a national ID number (In the U.S., social security numbers) Copyright 2005 Prentice-Hall

Figure 1-11: Internets Networks are connected by devices called routers Switches provide connections within networks, while routers provide connections between networks in an internet. Frames and Packets In single networks, message are called frames In internets, messages are called packets Understanding these two distinctions is important to avoid a lot of confusion this term. <Read the slide. Each major bullet is a build.> Copyright 2005 Prentice-Hall

Figure 1-11: Internets Packets are carried within frames
One packet is transmitted from the source host to the destination host across the internet Its IP destination address is that of the destination host <At this point, students should have a copy of the PDF version of Figure 1-12.> <Read this slide through. Explain that several things in it will be explained later. Students should come back to it later to really understand its ideas deeply.> WAN LAN LAN Router Copyright 2005 Prentice-Hall

Figure 1-11: Internets Packets are carried within frames
In each network, the packet is carried in (encapsulated in) a frame If there are N networks between the source and destination hosts, there will be one packet and N networks between the source and destination hosts, there will be one packet and N frames for a transmission <At this point, students should have a copy of the PDF version of Figure 1-12.> <Read this slide through. Explain that several things in it will be explained later. Students should come back to it later to really understand its ideas deeply.> WAN LAN LAN Router Copyright 2005 Prentice-Hall

Figure 1-12: Internet with Three Networks
Host A R1 Packet Network X A packet goes all the way across the internet; It’s path is its route Network Y Route A-B Network Z This slide shows a simplified version of Figure 1-12. It shows that there are three networks: Networks X, Y, and Z. It shows that a packet is a message that goes all the way across the internet From Host A in Network X to Host B in Network Z. The path the packet takes across the internet— Host A, Router R1, Router R2, and then Host B– is the packet’s route. R2 Host B Copyright 2005 Prentice-Hall

In Network X, the Packet is Placed in Frame X Frame X Details in Network X Packet Switch Host A AB-23-D1-A8-34-DD Switch Server Host Data link A-R1 A data Link is a frame’s path through its single network Switch X1 A route is a packet’s path through the internet This slide looks at what happens in Network X. <Before the build, note that the packet is carried in Frame X—a frame suitable for Network X’s technology.> <Go through the build. The first box introduces a data link as the frame’s path through a single network. This is called Data link A-R1 because it carries a frame from Host A to Router R1.> <Next, the slide shows part of the route that connects Host A to Host B all the way across the internet.> Mobile Client Host Switch X2 Router R1 D6-EE-92-5F-C1-56 Route A-B Network X Copyright 2005 Prentice-Hall

Details in Network Y To Network X Route A-B Router R1 Frame Y Data Link R1-R2 Packet This slide looks at what happens in Network Y. Router R1 takes the packet out of Frame X. It places the packet in a new frame, Frame Y. Frame Y is suitable for Network Y’s technology. Router R1 sends the frame to Router R2. To Network Z Router R2 AF-3B-E B5 Network Y Copyright 2005 Prentice-Hall

Network Z Details in Network Z Frame Z Packet Data Link R2-B Switch Z1 Host B 55-6B-CC-D4-A7-56 Switch Router R2 Switch Z2 Now we are in Network Z. Router R2 receives Frame Y. Router R2 removes the packet from Frame Y. Router R2 places the packet in a new frame, Frame Z. This frame is suitable for Network Z’s technology. Router R2 sends Frame Z to Host B. Host B removes the packet from Frame Z. The packet is now delivered. The internet has fulfilled its function. Switch Router Mobile Client Host Mobile Client Computer Copyright 2005 Prentice-Hall

In this internet with three networks, in a transmission, There is one packet There are three frames (one in each network) If a packet in an internet must pass through 10 networks, How many packets will be sent? How many frames must carry the packet? <This slide is a build.> <The first build text describes the figure we have just been seeing (Figure 1-12).> <The second asks the student to apply this to another network. Answers: There will be one packet. There will be 10 frames.> Copyright 2005 Prentice-Hall

Figure 1-13: Converting IP Addresses into Dotted Decimal Notation
IP Address (32 bits long) Divided into 4 bytes. These are segments. Convert each byte to decimal (result will be between 0 and 255)* 128 171 17 13 Dotted decimal notation (4 segments separated by dots) <Go through the table one row at a time.> In the first row, note that internet addresses are called IP addresses. [The Internet Protocol is the standard governing routers and packet transmission in most internets.] IP addresses always are 32 bits long. In the second row, note that the next step is to divide the 32-bit IP address into four 8-bit pieces. These pieces are called segments. In the third row, each segment has to be converted to decimal. We will see how to do this in the following figures. In the fourth row, the four segments in decimal are placed together, separated by dots. The IP address is now in dotted decimal notation. *The conversion process is described in the Hands On section at the end of the chapter. Copyright 2005 Prentice-Hall

Figure 1-25: Windows Calculator
2. Choose View, Scientific 3. Click on Bin to Indicate that the Source number Is binary. 4. Enter the bits of an 8-bit segment (The calculator has an 8-bit limit) You can convert binary to decimal and decimal to binary with the Windows Calculator. <Read through the build. If possible, demonstrate by bringing up the Windows Calculator and working through the example.> 1. Windows Calculators is under Programs  Accessories Copyright 2005 Prentice-Hall

Figure 1-25: Windows Calculator
6. See the result 5. Click on Dec To do the conversion <Read through the build.> Copyright 2005 Prentice-Hall

Converting Decimal to Binary
Click on Dec to indicate that the input is decimal Type a decimal number between 0 and 255 Click on Bin to do the conversion The result must be eight bits long to be a segment of an IP address So if the calculator shows 1100, the correct answer is <Read the slide.> Copyright 2005 Prentice-Hall

Figure 1-17: The Internet 1. Webserver Host Computer 1. User PC Host
3. Internet Backbone (Multiple ISP Carriers) Access Line Access Line Router NAP NAP ISP ISP ISP NAP <Read the build.> 1. Note again that all computers attached to the Internet are host computers. 2. To use the Internet, you must have an Internet service provider (ISP) Your ISP receives outgoing packets from you and sends incoming packets to you. ISPs also carry your packets across the Internet. ISPs also collect money to pay for the Internet. [The Internet is not free. It is a profit-making enterprise for the ISPs that provide service.] 3. The Internet backbone actually consists of many ISPs. [You might note that nobody owns or manages the Internet. Rather, the Internet is a collection of independent commercial ISPs. In nearly all countries, there is no government ownership at all. If this seems strange, this is exactly how the worldwide public switched telephone network has worked for several years.] ISPs interconnect at Network access points (NAPs) to exchange cross-ISP traffic ISP 2. User PC’s Internet Service Provider 2. Webserver’s Internet Service Provider 4. NAPs = Network Access Points Connect ISPs Copyright 2005 Prentice-Hall

Figure 1-18: Subnets in an Internet
LAN 2 LAN 1 LAN Subnet x Router R1 LAN Subnet 10.1.x.x Router R4 LAN Subnet x LAN Subnet 10.2.x.x LAN Subnet x WAN Subnet 123.x.x.x LAN Subnet 10.3.x.x LAN Subnet x One important piece of terminology is the concept of subnets. Internet professionals call single networks within internets subnets. Often just show subnets as lines in internet diagrams (Figure 1-19). But they are full networks with many switches and trunk lines. Router R3 Router R2 Note: Subnets are single networks (collections of switches, transmission lines) Often drawn as simple lines to focus on routers for internetworking Copyright 2005 Prentice-Hall

Figure 1-19: Terminology Differences for Single-Network and Internet Professionals
By Single-Network Professionals By Internet Professionals Single Networks Are Called Networks Subnets Internets Are Called Internets Networks Internet specialists and single-network specialists use conflicting terminology. [Historically, they came from different technical groups and developed different terminology.] To single-network professionals, internetworking is an extension to networks. To internetwork professionals, single networks are mere “subnets.”] <Read the two columns.> In this book, we will call internets “internets” and subnets “single networks.” In this book, we will usually call internets “internets” and subnets “single networks” Copyright 2005 Prentice-Hall

Figure 1-14: The Internet, internets, Intranets, and Extranets
Lower-case internet Any internet Upper-case Internet The global Internet Intranet An internet restricted to users within a single company Extranet A group of resources that can be accessed by authorized people in a group of companies A concept related to internets is “intranets.” <Read the slide.> Note that these intranets use the same standards that govern the global Internet—the TCP/IP standards [We will see these standards in the next chapter and in greater detail in Chapter 8.] [Historically, the intra- and extra- terminology was an was created by marketers If you find it confusing, imagine the problems of people who speak another language!] Copyright 2005 Prentice-Hall

Figure 1-20: IP Address Management
Every Host Must Have a Unique IP address Server hosts are given static IP addresses (unchanging) Clients get dynamic (temporary) IP addresses that may be different each time they use an internet Dynamic Host Configuration Protocol (DHCP) (Figure 1-21) Clients get these dynamic IP addresses from Dynamic Host Configuration Protocol (DHCP) servers (Figure 1- 21) Every host must have an IP address. [Just as every telephone needs a telephone number.] Servers get static (unchanging) IP addresses. [They need an unchanging address so that clients can find them. Imagine how hard it would be to find a store that moved each day to a different street address] Clients get dynamic (temporary) addresses when they start to use the Internet. [They may get a different IP address each time the start using the Internet. This is OK because nobody needs to find them; when clients use a server, the client packets tell the server their IP addresses.] There is a standard for giving clients dynamic IP addresses. This is the Dynamic Host Configuration Protocol (DHCP). Copyright 2005 Prentice-Hall

Figure 1-21: Dynamic Host Configuration Protocol (DHCP)
1. DHCP Request Message: “My 48-bit Ethernet address is A3-4E-CD F”. Please give me a 32-bit IP address.” 2. Pool of IP Addresses Client PC A3-4E-CD F DHCP Server <Click through the slide.> 1. In DHCP, the client broadcasts a DHCP request message to the DHCP server. This DHCP request message gives the client’s 48-bit Ethernet address (if it is on an Ethernet network). The message asks for a dynamic 32-bit IP address. 2. The DHCP server has a pool of available IP addresses. It selects one. 3. The DHCP response sends back an IP address for the client to use. As we will see in Chapter 8, it also sends other network configuration parameters. From now until the computer stops using the Internet, the IP address is its own. [Actually, IP addresses come with lease times. If the lease runs out before the client stops using the Internet, the client must begin the DHCP process again to get a new (sometimes the same) IP address.] 3. DHCP Response Message: “Computer at A3-4E-CD F, your 32-bit IP address is ”. (Usually other configuration parameters as well.) Copyright 2005 Prentice-Hall

Figure 1-20: IP Address Management
Domain Name System (DNS) (Figure 1-22) IP addresses are official addresses on the Internet and other internets Hosts can also have host names (e.g., cnn.com) Not official—like nicknames If you only know the host name of a host that you want to reach, your computer must learn its IP address DNS servers tell our computer the IP address of a target host whose name you know. (Figure 1-22) <Read the slide.> Copyright 2005 Prentice-Hall

Figure 1-22: The Domain Name System (DNS)
1. Client Host wishes to reach Voyager.cba.hawaii.edu; Needs to know its IP Address DNS Table Host Name IP Address … … Voyager.cba.hawaii.edu 2. Sends DNS Request Message “The host name is Voyager.cba.hawaii.edu” Local DNS Host <Click through the build.> 1. The client host wishes to reach a target server—Voyager.cba.hawaii.edu ( ). It only knows the host name (Voyager.cba.hawaii.edu). It needs to learn the IP address ( ). 2. (At the click) The client sends a Domain Name System (DNS) request message to its local DNS host. Voyager.cba.hawaii.edu Copyright 2005 Prentice-Hall

DNS Table 3. DNS Host looks up the target host’s IP address Host Name IP Address … … Voyager.cba.hawaii.edu DNS Host 4. DNS Response Message “The IP address is ” The Local DNS host notes that Voyager.cba.hawaii.edu has the IP address (Click) The DNS response message sends the IP address of Voyager… to the client. (Click) Afterward, the client can send packets to the target host, (Voyager…). 5. Client sends packets to Voyager.cba.hawaii.edu Copyright 2005 Prentice-Hall

The local DNS host sends back the response; the user is unaware that other DNS hosts were involved DNS Table Host Name IP Address … … Voyager.cba.hawaii.edu Client Host Local DNS Host 1. DNS Request Message 3. DNS Response Message What happens if the local DNS server does not know the IP address? As before, the client sends a DNS request message to its local DNS host. If the DNS host does not know the host name, it contacts other DNS hosts. One sends back the IP address. (Click). In any case, it is the local DNS host that sends the DNS response message back to the client Never the other DNS host. <Ask what will students think will happen if the Domain Name System cannot find the IP address.> <Answer: The Local DNS server’s response message will contain an error notification rather than the IP address.> 2. Request & Response If local DNS host does not have the target host’s IP address, it contacts other DNS hosts to get the IP address Anther DNS Host Copyright 2005 Prentice-Hall

Part VI: Security Perhaps the most pressing aspect of networks today is security. In this last section, we will take a brief look at key issues in security. We will then look at security in subsequent chapters. We will focus on security throughout the book, especially in Chapter 9. Copyright 2005 Prentice-Hall

Figure 1-23: Firewall and Hardened Hosts
Allowed Legitimate Packet Border Firewall Attacker The Internet Hardened Server Border firewall should pass legitimate packets Legitimate Packet Hardened Client PC One key security protection is the border firewall, which sits at the border between the local network and the Internet. The border firewall inspects all packets coming in from the Internet. We see that when the border firewall inspects a legitimate packet, it should let the packet pass. <The next slide looks at attack packets.> Legitimate Host Internal Corporate Network Log File Copyright 2005 Prentice-Hall

Server Border firewall should deny (drop) and log attack packets Attack Packet Border Firewall Attacker The Internet Hardened Client PC Denied Attack Packet However, when the firewall finds a provable attack packet, it drops the packet. It also logs the packet (records information about the dropped packet in a log file). Legitimate Host Internal Corporate Network Log File Copyright 2005 Prentice-Hall

Server Attack Packet Attack Packet Border Firewall Attacker The Internet Hosts should be hardened against attack packets that get through Attack Packet Denied Attack Packet Hardened Client PC Border firewalls never stop all attack packets. Consequently, some attack packets inevitably will reach internal clients and servers. All internal clients and servers need to be “hardened” against attacks. Chapter 9 discusses some ways to do so. For example, You can add active antivirus programs and personal firewalls to your PC. Legitimate Host Internal Corporate Network Log File Copyright 2005 Prentice-Hall

Figure 1-24: Cryptographic Protections
Cryptography The use of mathematical operations to thwart attacks on message dialogues between pairs of communicating parties (people, programs, or devices) Initial Authentication Determine the other party’s identity to thwart impostors When many people think of security, they think of cryptography. Cryptography (Crypto) is the use of mathematical operations to thwart attacks on message dialogues between pairs of communicating parties (people, programs, or devices). Notice that crypto protects message dialogues—the messages traveling between communication partners. This is different from stopping attack packets aimed at networks. Cryptography is expensive in terms of hardware, software, and management time. Consequently usually only sensitive dialogues are secured cryptographically. Cryptographic protection begins with authentication—requiring each partner to prove its identity. This prevents impostors from claiming to be someone else. [After all, computers and application programs cannot see one another.] Copyright 2005 Prentice-Hall

Figure 1-24: Cryptographic Protections
Message-by-Message Protections Encryption to provide confidentiality so that an eavesdropper cannot reach intercepted messages Electronic signatures provide message-by-message authentication to prevent the insertion of messages by an impostor after initial authentication Electronic signatures usually also provide message integrity; this tells the receiver whether anyone has changed the message en route After authentication, each message must be protected. Encryption for confidentiality prevents attackers from reading messages even if they intercept the messages. Encrypted messages look like random strings of ones and zeros. Of course, the receiver can decrypt the message, making it readable again. Also, each message is given an electronic signature—a string of bits following the message. The electronic signature proves the sender’s identity. In addition, if the message is tampered with en route, the message’s lack of integrity will become apparent. <Thought Question: You might ask the class why two forms of authentication—initial and message-by-message—are done. Answer: Without initial authentication, the session should not proceed. Message-by-message authentication solves a different problem. It prevents an impostor from succeeding in slipping in an inauthentic message after initial authentication.> Copyright 2005 Prentice-Hall

Network Elements: Recap
Applications (the only element that users care about) Computers Clients Servers Switches and Routers Transmission Lines Trunk lines Access Lines Messages (Frames) Wireless Access Points Never talk about an innovation “reducing cost,” “increasing speed,” etc. without specifying which element is cheaper or faster. For example, multiplexing only reduces the cost of trunk lines; other costs are not decreased Copyright 2005 Prentice-Hall

Recap: LANs and WANs LANs transmit data within corporate sites
WANs transmit data between corporate sites Each LAN or WAN is a single network LAN costs are low and speeds are high WAN costs are high and speeds are lower WAN Copyright 2005 Prentice-Hall

Recap: Internets Most firms have multiple LANs and WANs.
They must create internets An internet is a collection of networks connected by routers so that any application on any host on any single network can communicate with any application on any other host on any other network in the internet. Application Application WAN LAN LAN Router Router Copyright 2005 Prentice-Hall

Recap: Internets Elements of an Internet
Computers connected to the internet are called hosts Both servers and client PCs are hosts Routers connect the networks of the internet together In contrast, switches forward frames within individual networks Router Router WAN LAN LAN Client PC Host Server Host Copyright 2005 Prentice-Hall

Recap: Internets Hosts Have Two Addresses IP Address
This is the host’s official address on its internet 32 bits long Expressed for people in dotted decimal notation (e.g., 128, 171, 17.13) Single Network Addresses This is the host’s address on its single network Ethernet addresses, for instance, are 48 bits long Expressed in hexadecimal notation, e.g., AF-23-9B-E Copyright 2005 Prentice-Hall

Recap: Internets Switches versus Routers Messages
Switches move frames through a single network (LAN or WAN) Routers move packets through internets Messages Messages in single networks are called frames Messages in internets are called packets Packets are encapsulated within (carried inside) frames Copyright 2005 Prentice-Hall

Recap: Security Security Firewalls Hardened Hosts
Cryptographic security for sensitive dialogues Initial authentication Encryption for confidentiality Electronic signatures for authentication and message integrity Copyright 2005 Prentice-Hall

An Introduction to Networking

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