1. An Introduction to Networking Chapter 1 Panko’s Business Data Networks and Telecommunications, 6 th edition Copyright 2007 Prentice-Hall May only be used by adopters of the book

2. 2 The Chapter This chapter is a survey of the key concepts we will see in this course The rest of the book essentially fleshes out the concepts we will see in this chapter

3. Part I: Basic Networks Concepts Concepts we will see throughout the book

4. 4 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. Network

5. 5 Figure 1-2: The Internet and Applications The Internet (Global Transmission System To Carry Application Data) The Internet (Global Transmission System To Carry Application Data) Client Computer Mobile Client Computer Browser E-Mail Client Webserver Application Server Computer (Webserver) E-Mail Application Server Computer (Mail Server) The Internet is a Global TRANSMISSION System Applications are DELIVERED By The Internet

6. 6 Figure 1-1: Basic Networking Concepts The Internet –Client/server applications PC clients receive service from servers Many C/S applications need special clients Many (but not all) C/S applications only need a browser Client Computer Server Computer Server ProgramClient Program

7. 7 Figure 1-1: Basic Networking Concepts Internal Corporate Networks –For transmission among computers within a corporation –Transaction processing applications High-volume clerical applications Accounting, payroll, billing, etc. –Voice over IP (VoIP)

8. 8 Figure 1-1: Basic Networking Concepts Data Communications and Telecommunications –Data communications, as the name suggests, involves the transmission of data (text, numbers, pictures, and other information). –In turn, telecommunications is the transmission of voice and video, including ordinary telephony and broadcast and cable television. –Beginning to converge

9. 9 Figure 1-1: Basic Networking Concepts Digital Transmission –Information is first converted into a string of ones and zeros (binary) –Next, the ones and zeros are converted into signals that propagate over transmission media. –More detail in Chapter 3 Hello… 12345… 101001 1000100

10. 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

11. 11 Figure 1-3: Elements of a Network Wireless Access Point Mobile Client Router Outside World Server Computer Client Computer Switch 1 Switch 2 Switch 3 Message (Frame) Access Line Trunk Line Server ApplicationClient Application Networks connect applications applications on different computers. Applications are all users care about Networks connect applications applications on different computers. Applications are all users care about

12. 12 Figure 1-3: Elements of a Network Wireless Access Point Mobile Client Router Outside World ServerComputer ClientComputer Switch 1 Switch 2 Switch 3 Message (Frame) Access Line Trunk Line Server ApplicationClient Application Networks connect computers: clients (fixed and mobile) and servers Networks connect computers: clients (fixed and mobile) and servers

13. 13 Figure 1-3: Elements of a Network Wireless Access Point Mobile Client Router Outside World Server Computer Client Computer Switch 1 Switch 3 Message (Frame) Trunk Line Server ApplicationClient Application The path a frame takes is called its data link The path a frame takes is called its data link Computers (and routers) usually communicate by sending messages called frames Computers (and routers) usually communicate by sending messages called frames Data Link

14. 14 Figure 1-3: Elements of a Network Wireless Access Point Mobile Client Router Outside World Server Computer Client Computer Switch 4 Message (Frame) Trunk Line Server ApplicationClient Application Switch 2 Switch 1 Switch 3 Frame to Sw1 Frame to Sw1 Frame to Sw2 Frame to Sw2 Frame To Sw3 Frame To Sw3 Frame to Server Frame to Server Switches Switches Forward Frames Sequentially Switches Switches Forward Frames Sequentially

15. 15 Figure 1-5: Ethernet Switch Operation A1-44-D5-1F-AA-4CB2-CD-13-5B-E4-65 Switch D4-47-55-C4-B6-F9 C3-2D-55-3B-A9-4F Port 15 Frame to C3… A1- sends a frame to C3- Frame to C3… Switch sends frame to C3- Switching Table PortHost 10A1-44-D5-1F-AA-4C 13B2-CD-13-5B-E4-65 15C3-2D-55-3B-A9-4F 16 D4-47-55-C4-B6-F9 Switching Table PortHost 10A1-44-D5-1F-AA-4C 13B2-CD-13-5B-E4-65 15C3-2D-55-3B-A9-4F 16 D4-47-55-C4-B6-F9 15C3-2D-55-3B-A9-4F C3- is out Port 15 1 2 3

16. 16 Figure 1-3: Elements of a Network Small Switches (Stacked): Large Switch Both sizes of switches are 48 cm (19 inches) wide

17. 17 Figure 1-3: Elements of a Network Wireless Access Point Mobile Client Router Outside World Server Computer Client Computer Switch 1 Switch 2 Switch 3 Switch 4 Message (Frame) Access Line Trunk Line Server ApplicationClient Application Wireless Access Points Points Connect Wireless Stations to Switches Wireless Access Points Points Connect Wireless Stations to Switches

18. 18 Figure 1-3: Elements of a Network Wireless Access Point Mobile Client Router Outside World Server Computer Client Computer Switch 1 Switch 2 Switch 3 Switch 4 Message (Frame) Access Line Trunk Line Server ApplicationClient Application Routers connect networks to the outside world; Treated just like computers in single networks Routers connect networks to the outside world; Treated just like computers in single networks

19. 19 Figure 1-3: Elements of a Network Wireless Access Point Mobile Client Router Outside World Server Computer Client Computer Switch 1 Switch 2 Switch 3 Switch 4 Message (Frame) Access Line Trunk Line Server ApplicationClient Application Access Lines Connect Computers to Switches Access Lines Connect Computers to Switches

20. 20 Figure 1-3: Elements of a Network Wireless Access Point Mobile Client Router Outside World Server Computer Client Computer Switch 1 Switch 2 Switch 3 Switch 4 Message (Frame) Access Line Trunk Lines Server ApplicationClient Application Trunk Lines Trunk Lines Connect Switches to Switches and Switches to Routers Trunk Lines Trunk Lines Connect Switches to Switches and Switches to Routers Trunk Line

21. 21 Figure 1-4: Packet Switching and Multiplexing Client Computer A Mobile Client Computer B Router D Server Computer C AC BD Access Line Trunk Line Multiplexing Mixes the Messages of Multiple Conversations on a Trunk Line So Packet Switching Reduces the Cost of Trunk Lines Breaking Communications into Small Messages is Called Packet Switching, even if the Messages are Frames

22. Part III: Transmission Speed

23. 23 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) 10 3 10 6 10 9 10 12

24. 24 Figure 1-6: Transmission Speed Measuring Transmission Speed –What is 23,000 bps in metric notation? –What is 3,000,000,000 bps in metric notation? –What is 15,100,000 bps in metric notation? Occasionally measured in bytes per second Written as Bps

25. 25 Figure 1-6: Transmission Speed Writing Transmission Speeds –The rule for writing speeds (and metric numbers in general) 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).

26. 26 Figure 1-6: Transmission Speed Writing Transmission Speeds –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 Multiple the metric suffix Mbps by 1,000 –Gbps Result: –1.2 Gbps –When you make either the number or the metric smaller, you have to make the other one bigger

27. 27 Figure 1-6: Transmission Speed Writing Transmission Speeds –How to convert 0.036 Mbps to proper form Multiply the number 0.036 by 1000 –Move decimal point three places to the right: 36 Divide the metric suffix Mbps by 1,000 –kbps Result: –36 kbps

28. 28 Figure 1-6: Transmission Speed Writing Transmission Speeds –How should you write the following in proper form? 549.73 kbps 0.47 Gbps 11,200 Mbps.0021 Gbps

29. 29 Figure 1-6: Transmission Speed Rated Speed Versus Throughput –Rated speed is the speed a network should provide, based on standards –Throughput is the speed a network actually provides –We will use this distinction constantly throughout this book

30. 30 Figure 1-6: Transmission Speed Rated Speed Versus Throughput –When transmission capacity is shared by multiple users, The total shared throughput is the aggregate throughput Individual throughput is what individuals receive as a fraction of the aggregate throughput

31. Part IV: LANs and WANs

32. 32 First Bank of Paradise (FBP) The book’s running case study –Composite mid-size bank in Hawaii –Banks are fairly “typical” firms, although they have stronger need for security –Warren Chun is the chief information officer (CIO) –Yvonne Champion is the network manager

33. 33 First Bank of Paradise (FBP) Annual Revenues: $4.5 Billion Operations –60 Branches –375 ATMs (Automated Teller Machines) Network –700 Ethernet switches –450 Routers

34. 34 First Bank of Paradise (FBP) Computers –2,300 desktop and notebook user PCs –130 Windows servers –60 Unix servers Information Systems Staff –112 people

35. 35 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 within a site. Campus, building, and SOHO (Small Office or Home Office) LANs For transmission between sites For transmission between sites Cost per bit Transmitted Low High Typical Speed Unshared 100 Mbps to a gigabit per second to each desktop. Even faster trunk line speeds. 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 Shared 128 kbps to several megabits per second trunk line speeds

36. 36 Figure 1-8: LANs Versus WANs Characteristics Management LANs WANs On own premises, so firm builds and manages its own LAN or outsources the Work 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. 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 Only those offered by carrier

37. 37 Figure 1-9: Local Area Network (LAN) in a Large Building Multi-floor Office Building The bank has multiple LANs—one at each site The bank has multiple LANs—one at each site

38. 38 Figure 1-9: Local Area Network (LAN) in a Large Building Router Core Switch Workgroup Switch 2 Workgroup Switch 1 Wall Jack To WAN Wall Jack Server Client 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

39. 39 Figure 1-10: Workgroup Switch (19 inches / 48 cm Wide) 48 cm (19 in.) Workgroup Switch with 16 ports Wire cord going out to a computer or to another switch Wire cord going out to a computer or to another switch

40. 40 Figure 1-7: The First Bank of Paradise’s Wide Area Networks (WANs) Operations Headquarters North Shore OC3 Private Leased Line T3 Bank has multiple facilities connected by multiple WANs Bank has multiple facilities connected by multiple WANs Frame Relay Network Branch Office

41. Part V: Internets

42. 42 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. LAN WAN LAN Application Router

43. 43 Figure 1-11: Internets Internet Components –All computers in an internet are called hosts Servers, clients, PDAs, cellphones, etc. Cat Internet Client PC (Host) Cellphone (Host) VoIP Phone (Host) PDA (Host) Server (Host)

44. 44 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., 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- E8-67-47)

45. 45 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

46. 46 Figure 1-11: Internets Packets are carried within frames –One packet is transmitted from the source host to the destination host Its IP destination address is that of the destination host –In each network, the packet is carried in (encapsulated in) a frame (Figure 1-12) –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

47. 47 Figure 1-12: Internet with Three Networks Host B Host A Network X Network Y Network Z R1 R2 Route A-B Packet A packet goes all the way across the internet; It’s path is its route A packet goes all the way across the internet; It’s path is its route

48. 48 Figure 1-12: Internet with Three Networks Messages in single networks (LANs or WANs) are called frames Message in internets are called packets –Travel from the source host to the destination host across the entire internet Within a single network, the packet is encapsulated in (carried in) the network’s frame Frame Truck (frame) Package (Packet) Packet

49. 49 Figure 1-12: Internet with Three Networks Mobile Client Host Server Host Switch X2 Switch X1 Switch Router R1 D6-EE-92-5F-C1-56 Network X Route A-B A route is a packet’s path through the internet A route is a packet’s path through the internet Details in Network X Details in Network X Data link A-R1 Data link A-R1 A data Link is a frame’s path through its single network A data Link is a frame’s path through its single network In Network X, the Packet is Placed in Frame X Packet Frame X Host A 10.0.0.23 AB-23-D1-A8-34-DD

50. 50 Figure 1-12: Internet with Three Networks Router R1 Router R2 AF-3B-E7-39-12-B5 Packet Frame Y To Network X To Network Z Network Y Data Link R1-R2 Route A-B Details in Network Y Details in Network Y

51. 51 Figure 1-12: Internet with Three Networks Host B www.pukanui.com 1.3.45.111 55-6B-CC-D4-A7-56 Mobile Client Host Switch Z1 Switch Z2 Switch Packet Frame Z Network Z Router R2 Router Data Link R2-B Details in Network Z Details in Network Z Mobile Client Computer

52. 52 Figure 1-12: Internet with Three Networks 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?

53. 53 Figure 1-12: Internet with Three Networks Spelled in lowercase, “internet” is any internet Spelled in uppercase, “Internet” is the global Internet

54. 54 10000000101010110001000100001101 Figure 1-13: Converting IP Addresses into Dotted Decimal Notation Divided into 4 bytes. These are segments. 100000001010101100010001 00001101 Dotted decimal notation (4 segments separated by dots) Dotted decimal notation (4 segments separated by dots) IP Address (32 bits long) Convert each byte to decimal (result will be between 0 and 255)* 12817117 13 *The conversion process is described in the Hands On section at the end of the chapter. 128.171.17.13

55. 55 Figure 1-25: Windows Calculator 1. Open Calculator, which is in the Program Group Accessories 1. Open Calculator, which is in the Program Group Accessories 2. Select View, Scientific 2. Select View, Scientific

56. 56 Figure 1-25: Windows Calculator 3. Enter initial data type here Dec = Decimal Bin = Binary 3. Enter initial data type here Dec = Decimal Bin = Binary 4. Enter data on keypad (Limit is 8 bits for Binary) 4. Enter data on keypad (Limit is 8 bits for Binary)

57. 57 Figure 1-25: Windows Calculator 6. Observe answer Initial zeros are dropped, so answer is 0001 0111 6. Observe answer Initial zeros are dropped, so answer is 0001 0111 5. Enter final data type here, observe results 5. Enter final data type here, observe results

58. 58 Figure 1-14: The Internet, internets, Intranets, and Extranets The Global Internet –As noted earlier, Spelled with a lowercase i, internet means any internet Spelled with a uppercase I, Internet means the global Internet

59. 59 Figure 1-14: The Internet, internets, Intranets, and Extranets The Internet (Figure 1-18) –Host computers –Internet service providers (ISPs) Required to access the Internet Carry your packets across the Internet Collect money to pay for the Internet –The Internet backbone consists of many ISPs ISPs interconnect at Network access points (NAPs) to exchange cross-ISP traffic

60. 60 Figure 1-17: The Internet User PC’s Internet Service Provider Webserver’s Internet Service Provider ISP User PC Host Computer Webserver Host Computer NAP = Network Access Point Router NAP ISP Internet Backbone (Multiple ISP Carriers) Access Line Access Line

61. 61 Figure 1-17: The Internet User PC’s Internet Service Provider Webserver’s Internet Service Provider ISP User PC Host Computer Webserver Host NAP = Network Access Point Router NAP ISP Internet Backbone (Multiple ISP Carriers) Access Line Access Line

62. 62 Figure 1-18: Subnets in an Internet LAN 1 LAN 2 LAN Subnet 10.1.x.x WAN Subnet 123.x.x.x LAN Subnet 60.4.3.x LAN Subnet 10.2.x.x LAN Subnet 10.3.x.x LAN Subnet 60.4.15.x LAN Subnet 60.4.7.x Note: Subnets are single networks (collections of switches, transmission lines) Router R1 Router R3 Router R4 Router R2 LAN Subnet 60.4.131.x

63. 63 Figure 1-19: Terminology Differences for Single- Network and Internet Professionals By Single-Network Professionals By Internet Professionals By Internet Professionals Single Networks Are Called Networks Subnets Internets Are Called Internets Networks In this book, to avoid confusion, we will call internets “internets” and subnets “single networks”

64. 64 Figure 1-14: The Internet, internets, Intranets, and Extranets Intranets –An intranet is an internal internet for use within an organization –Based on the TCP/IP standards created for the Internet “Intra” means “within”

65. 65 Figure 1-14: The Internet, internets, Intranets, and Extranets Extranets –To connect multiple firms Only some computers from each firm are on the extranet –Use TCP/IP standards “Extra” means “outside”

66. 66 Figure 1-14: The Internet, internets, Intranets, and Extranets Intranets, Extranets, and the Internet –Confusingly, both intranets and extranets can use the Internet for some of their transmission capacity

67. 67 Figure 1-15: Routers (19 inches / 48 cm Wide)

68. 68 Figure 1-16: Small Router for a Branch Office (19 inches / 48 cm Wide)

69. 69 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)

70. 70 Figure 1-21: Dynamic Host Configuration Protocol (DHCP) Client PC A3-4E-CD-59-28-7F DHCP Server DHCP Request Message: “My 48-bit Ethernet address is A3-4E-CD-59-28-7F”. Please give me a 32-bit IP address.” Pool of IP Addresses

71. 71 Figure 1-21: Dynamic Host Configuration Protocol (DHCP) Client PC A3-4E-CD-59-28-7F DHCP Server DHCP Response Message: “Computer at A3-4E-CD-59-28-7F, your 32-bit IP address is 11010000101111101010101100000010”. (Usually other configuration parameters as well.) Pool of IP Addresses

72. 72 動態主機組態協定 (DHCP) Dynamic Host Configuration Protocol 自動設定電腦的 –IP 位址 (163.22.20.223) – 子網路遮罩 (255.255.255.0) – 預設通訊閘 (163.22.20.254) – 領域名稱伺服器 (163.22.2.1) –… winipcfg (Win 98/Me) ipconfig /all (Win 2000/XP)

73. 73 1 2 3 控制台  網路和網際網路連線

74. 74

75. 75 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)

76. 76 Figure 1-22: The Domain Name System (DNS) Host NameIP Address…… Voyager.cba.hawaii.edu128.171.17.13… Host NameIP Address…… Voyager.cba.hawaii.edu128.171.17.13… DNS Table 1. Client Host wishes to reach Voyager.cba.hawaii.edu; Needs to know its IP Address 2. Sends DNS Request Message “The host name is Voyager.cba.hawaii.edu” Voyager.cba.hawaii.edu 128.171.17.13 Local DNS Host

77. 77 Figure 1-22: The Domain Name System (DNS) Host NameIP Address…… Voyager.cba.hawaii.edu128.171.17.13… Host NameIP Address…… Voyager.cba.hawaii.edu128.171.17.13… DNS Table 4. DNS Response Message “The IP address is 128.171.17.13” Voyager.cba.hawaii.edu 128.171.17.13 5. Client sends packets to 128.171.17.13 3. DNS Host looks up IP address DNS Host

78. 78 Figure 1-22: The Domain Name System (DNS) Host NameIP Address…… Voyager.cba.hawaii.edu128.171.17.13… Host NameIP Address…… Voyager.cba.hawaii.edu128.171.17.13… DNS Table Client Host 1. DNS Request Message Anther DNS Host Local DNS Host 3. DNS Response Message The local DNS host sends back the response; the user is unaware that other DNS hosts were involved The local DNS host sends back the response; the user is unaware that other DNS hosts were involved If local DNS host does not have the target host’s IP address, it contacts other DNS hosts to get the IP address If local DNS host does not have the target host’s IP address, it contacts other DNS hosts to get the IP address 2. Request & Response

79. Part VI: Security

80. 80 Figure 1-23: Firewall and Hardened Hosts Legitimate Host Legitimate Packet Border Firewall Hardened Server Allowed Legitimate Packet Hardened Client PC Internal Corporate Network Border firewall should pass legitimate packets Border firewall should pass legitimate packets The Internet Attacker Log File

81. 81 Figure 1-23: Firewall and Hardened Hosts Legitimate Host Attack Packet Denied Attack Packet Hardened Server Hardened Client PC Internal Corporate Network Border firewall should deny (drop) and log attack packets Border firewall should deny (drop) and log attack packets The Internet Border Firewall Attacker Log File

82. 82 Figure 1-23: Firewall and Hardened Hosts Legitimate Host Attacker Attack Packet Denied Attack Packet Internal Corporate Network The Internet Border Firewall Hardened Server Hardened Server Hardened Client PC Hardened Client PC Attack Packet Attack Packet Log File Hosts should be hardened against attack packets that get through Hosts should be hardened against attack packets that get through

83. 83 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) Cryptography is Expensive –Usually only sensitive dialogues are cryptographically secured Initial Authentication –Determine the other party’s identity to thwart impostors

84. 84 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

85. Recap

86. 86 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 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

87. 87 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

88. 88 LAN WAN LAN 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 Router

89. 89 LAN WAN LAN 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 Client PC Host Server Host Router

90. 90 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- E8-67-47

91. 91 Recap Switches versus Routers –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

92. 92 Security –Firewalls –Hardened Hosts –Cryptographic security for sensitive dialogues Initial authentication Encryption for confidentiality Electronic signatures for authentication and message integrity