Summary 419 Routers running proxy ARP capture ARP packets. They respond with their MAC addresses for those requests in which the IP address is not in the range of addresses of the local subnet. For example, in Figure 7-49, if Machine A sends an ARP requests to Machine F, the router processes the request and replies to A with its own MAC address to be mapped to the IP address of Machine F. In the previous description of how data is sent to a host on a different subnet, the default gateway is configured. If the source host does not have a default gateway configured, it sends an ARP request. All hosts on the segment, including the router, receive the ARP request. The router compares the IP destination address with the IP subnet address to determine whether the destination IP address is on the same subnet as the source host. If the subnet address is the same, the router discards the packet. The packet is discarded because the destination IP address is on the same segment as the source’s IP address. This means that the destination device on the segment should respond to the ARP request. The exception to this is that the destination IP address is not currently assigned, which generates an error response on the source host. Figure 7-49 Proxy ARP Network Example Summary In this chapter, you learned the following key points: ■ To provide extra flexibility for the network administrator, networks—particularly large ones—are often divided into smaller networks called subnetworks or sub- nets. Subnetting allows a network administrator to get around IPv4’s limitations by dividing a single network address into many subnets visible only within that single network. ■ The function of a subnet mask is to tell devices which part of an address is the network number, including the subnet, and which part is the host. ■ Internetworking functions of the network layer include network addressing and best-path selection for data traffic. Router A B C D E F 1102.book Page 419 Tuesday, May 20, 2003 2:53 PM 420 Chapter 7: TCP/IP Protocol Suite and IP Addressing ■ How to explain IP addressing, IP address classes, reserved IP address space, private IP address space, and IP subnetting ■ How to explain the evolution of IP addressing, including the need for an increase in the size of the IP address space ■ A computer needs an IP address (hierarchical address) to communicate on the Internet. ■ There are two ways to configure IP addresses: statically and dynamically. ■ There are multiple ways to assign an IP address dynamically. ■ When a device does not know its own IP address, it uses RARP, BOOTP, or DHCP. ■ When the device that originated a RARP request receives a RARP reply, it copies its IP address into its memory cache, where it resides for as long as the session lasts. ■ BOOTP supplies more information to a client than just an IP address. ■ DHCP allows computers to be mobile, connecting to many different networks as needed. To supplement all you’ve learned in this chapter, refer to the chapter-specific Videos, PhotoZooms, and e-Lab Activities on the CD-ROM accompanying this book. 1102.book Page 420 Tuesday, May 20, 2003 2:53 PM Key Terms 421 Key Terms application layer Handles high-level protocols and issues of representation, encoding, and dialog control. The TCP/IP protocol suite combines all application-related issues into one layer and ensures that this data is properly packaged for the next layer. broadcast address Used to broadcast packets to all the devices on a network. Class A address Designed to support extremely large networks. A Class A IP address uses only the first octet to indicate the network address. The remaining three octets enumerate host addresses. Class B address Designed to support the needs of moderate- to large-sized networks. A Class B IP address uses two of the four octets to indicate the network address. The other two octets specify host addresses. Class C address The most commonly used of the original address classes. This address space was intended to support a lot of small networks. Class D address Created to enable multicasting in an IP address. Class E address The IETF reserves these addresses for its own research. Therefore, no Class E addresses have been released for use in the Internet. dotted-decimal format In this notation, each IP address is written as four parts separated by periods, or dots. IP address class A 32-bit IP address is broken into a network part and a host part. A bit or bit sequence at the start of each address determines the address’s class. IP Version 6 (IPv6) The replacement for the current version of IP (Version 4). IPv6 includes support for flow ID in the packet header, which can be used to identify flows. Formerly called IPng (IP next generation). multicast address A unique network address that directs packets that have that destination address to predefined groups of IP addresses. network access layer The layer that is concerned with all the issues that an IP packet requires to make a physical link to the network medium. subnetting The method of dividing full network address classes into smaller pieces. This has prevented complete IP address exhaustion. 1102.book Page 421 Tuesday, May 20, 2003 2:53 PM 422 Chapter 7: TCP/IP Protocol Suite and IP Addressing Transmission Control Protocol/Internet Protocol (TCP/IP) The common name for the suite of protocols developed by the U.S. DoD in the 1970s to support the construction of worldwide internetworks. TCP and IP are the two best-known protocols in the suite. transport layer Provides transport services from the source host to the destination host. It constitutes a logical connection between the network’s endpoints: the sending host and the receiving host. Review Questions Complete all the review questions to test your understanding of the topics and con- cepts in this chapter. Answers are listed in Appendix C, “Check Your Understanding Answer Key.” 1. What transport layer protocol does TFTP use? A. TCP B. IP C. UDP D. CFTP 2. Which of the following is a basic service of the transport layer? A. Provides reliability by using sequence numbers and acknowledgments B. Segments upper-layer application data C. Establishes end-to-end operations D. All of the above 3. Which of the following protocols operate at the TCP/IP Internet layer? A. IP B. ICMP C. ARP D. All of the above 4. What is the first thing that happens when a DHCP client boots? A. DHCPREQUEST B. DHCPBOOT C. DHCPDISCOVER D. None of the above 1102.book Page 422 Tuesday, May 20, 2003 2:53 PM Review Questions 423 5. How does the network layer forward packets from the source to the destination? A. By using a routing table B. By using ARP responses C. By referring to a name server D. By referring to the bridge 6. If a device doesn’t know the MAC address of a device on an adjacent network, it sends an ARP request to what? A. The default gateway B. The closest router C. The router interface D. All of the above 7. What is in a RARP request? A. A MAC header and the RARP request message B. A MAC header, a RARP header, and a data packet C. A RARP header and MAC and IP addresses D. A RARP header and an ARP trailer 8. What are the two parts of an IP address? A. Network address and host address B. Network address and MAC address C. Host address and MAC address D. MAC address and subnet mask 9. What Internet protocol is used to map a known IP address to an unknown MAC address? A. UDP B. ICMP C. ARP D. RARP 10. Which of the following initiates an ARP request? A. A device that can locate the destination IP address in its ARP table B. The RARP server in response to a malfunctioning device C. A diskless workstation with an empty cache D. A device that cannot locate the destination MAC address in its ARP table 1102.book Page 423 Tuesday, May 20, 2003 2:53 PM 424 Chapter 7: TCP/IP Protocol Suite and IP Addressing 11. Which of the following best describes an ARP table? A. A way to reduce network traffic by providing lists of shortcuts and routes to common destinations B. A way to route data within networks that are divided into subnetworks C. A protocol that performs an application layer conversion of information from one stack to another D. A section of RAM on each device that maps IP addresses to MAC addresses 12. Which of the following best describes the ARP reply? A. A device sends its MAC address to a source in response to an ARP request B. The shortest path between the source and the destination C. The updating of ARP tables through intercepting and reading messages traveling on the network D. The method of finding IP addresses based on the MAC address, used primarily by RARP servers 13. Why are current, updated ARP tables important? A. For testing links in the network B. For limiting the number of broadcasts C. For reducing network administrator maintenance time D. For resolving addressing conflicts 14. Why is a RARP request made? A. A source knows its MAC address but not its IP address. B. The data packet needs to find the shortest route between the destination and the source. C. The administrator needs to manually configure the system. D. A link in the network faults, and a redundant system must be activated. 15. Which of the following best describes TCP/IP? A. It is a suite of protocols that can be used to communicate across any set of interconnected networks. B. It is a suite of protocols that allows LANs to connect to WANs. C. It is a suite of protocols that allows for data transmission across a multitude of networks. D. It is a suite of protocols that allows different devices to be shared by intercon- nected networks. 1102.book Page 424 Tuesday, May 20, 2003 2:53 PM Review Questions 425 16. Which of the following does not describe the TCP/IP protocol stack? A. It maps closely to the OSI reference model’s upper layers. B. It supports all standard physical and data link protocols. C. It transfers information in a sequence of datagrams. D. It reassembles datagrams into complete messages at the receiving location. 17. The TCP/IP protocol suite has specifications for which layers of the OSI model? A. 1 through 3 B. 1 through 4 and 7 C. 3, 4, and 5 through 7 D. 1, 3, and 4 18. Which of the following is not a function of the network layer? A. RARP determines network addresses when data link layer addresses are known. B. ICMP provides control and messaging capabilities. C. ARP determines the data link layer address for known IP addresses. D. UDP provides connectionless exchange of datagrams without acknowledgments. 19. Which of the following is one of the protocols found at the transport layer? A. UCP B. UDP C. TDP D. TDC 1102.book Page 425 Tuesday, May 20, 2003 2:53 PM Objectives Upon completion of this chapter, you will be able to ■ Describe the purpose of routable/routed protocols such as IP ■ Understand the function of IP as a connectionless versus connection-oriented networking service ■ Explain how routers operate at the network layer ■ Describe how routed protocols define the format and use of the fields in a packet ■ Compare OSI Layer 2 LAN switching to Layer 3 routing ■ Explain how routing protocols are used between routers to determine paths and maintain routing tables ■ Explain how routers accomplish path selection and switching functions to transport packets through an internetwork ■ Describe the differences between static and dynamic routing ■ Describe the differences between distance-vector and link-state routing protocols, including convergence ■ Describe the differences between interior and exterior routing protocols, and cite examples of each ■ Understand the purpose and benefits of subnetting ■ Create subnets using the subnet mask based on requirements ■ Calculate the resident network through the ANDing process 1102.book Page 426 Tuesday, May 20, 2003 2:53 PM Chapter 8 Routing Fundamentals and Subnets This chapter covers topics related to the Internet Protocol (IP). IP is the fundamental pro- tocol used in the Internet. Topics discussed include how IP is delivered, how the header is modified at Layer 3 devices, and the actual layout of the IP packet. This chapter also covers the relationship between connectionless and connection-oriented networking services. You will learn the difference between routing and routed protocols and how routers track dis- tance between locations. This chapter introduces the distance-vector, link-state, and hybrid routing approaches and how each resolves common routing problems. Be sure to look at this chapter’s associated e-Lab Activities, Videos, and PhotoZooms, which you will find on the CD-ROM accompanying this book. These CD elements are designed to supplement the material and reinforce the concepts introduced in this chapter. Routed, Routable, and Routing Protocols A protocol is a standards-based set of rules that determines how computers communicate with each other across networks. A protocol also serves as the common denominator or medium by which different applications, hosts, or systems communicate. When computers communicate with one another, they exchange data messages. To accept and act on these messages, computers must have definitions of how a message is defined and what it means. Examples of messages include establishing a connection to a remote machine, sending or receiving e-mail, and transferring files and data. 1102.book Page 427 Tuesday, May 20, 2003 2:53 PM 428 Chapter 8: Routing Fundamentals and Subnets A protocol describes ■ The format that a message must take ■ The way in which computers must exchange a message within the context of a particular activity, such as sending messages across networks Because of the similarity of a routed/routable protocol versus a routing protocol, confusion over these terms often exists. The following provides some clarification: ■ Routed protocol—Any network protocol that provides enough information in its network layer address to allow a packet to be forwarded from one host to another host based on the addressing scheme. Routed protocols define the field formats within a packet. Packets are generally conveyed from end system to end system. A routed protocol uses the routing table to forward packets. Examples of routed or routable protocols are shown in the routing table in Figure 8-1. They include the following: — Internet Protocol (IP) — Internetwork Packet Exchange (IPX) — AppleTalk Figure 8-1 Routed/Routable Protocols Token Ring Token Ring Novell Apple Routing Tables IPX 3a.0800.5678.12ab IP 15.16.50.3 AppleTalk 100.110 IP 15.16.42.8 IP 15.17.132.6AppleTalk 200.167 IPX 4b.0800.0121.ab13 IP 1102.book Page 428 Tuesday, May 20, 2003 2:53 PM . Tables IPX 3a.0800.5678 . 12 ab IP 15 .16 .50.3 AppleTalk 10 0 .11 0 IP 15 .16 . 42. 8 IP 15 .17 .13 2. 6AppleTalk 20 0 .16 7 IPX 4b.0800. 0 12 1.ab13 IP 11 02. book Page 428 Tuesday, May 20 , 20 03 2: 53 PM . PhotoZooms, and e-Lab Activities on the CD-ROM accompanying this book. 11 02. book Page 420 Tuesday, May 20 , 20 03 2: 53 PM Key Terms 4 21 Key Terms application layer Handles high-level protocols and issues. the destination MAC address in its ARP table 11 02. book Page 423 Tuesday, May 20 , 20 03 2: 53 PM 424 Chapter 7: TCP/IP Protocol Suite and IP Addressing 11 . Which of the following best describes an