C H A P T E R 12 IP Addressing and Subnetting In Chapter 5, “Fundamentals of IP,” you learned about the basic concepts and terminology relating to IP addressing. These concepts were introduced early in the book because your understanding of many basic networking concepts depends on a base knowledge of IP addressing. In this chapter, you will learn about the concepts and mathematics that let you analyze IP addresses and subnets. IP addressing is the only major topic that happens to get coverage on both of the INTRO and ICND exams. To answer questions on either CCNA exam, you will need to discover the structure of IP addresses, list the addresses in the same subnet, list the other subnets of that same network, identify the numbers of hosts in a subnet, and identify other information about addresses and subnets. This chapter describes the math and processes used to answer these questions. This chapter also happens to cover a few topics related IP address scalability issues related to Internet growth. “Do I Know This Already?” Quiz The purpose of the “Do I Know This Already?” quiz is to help you decide whether you really need to read the entire chapter. If you already intend to read the entire chapter, you do not necessarily need to answer these questions now. The 14-question quiz, derived from the major sections in the “Foundation Topics” portion of the chapter, helps you determine how to spend your limited study time. Table 12-1 outlines the major topics discussed in this chapter and the “Do I Know This Already?” quiz questions that correspond to those topics. Table 12-1 “Do I Know This Already?” Foundation Topics Section-to-Question Mapping Foundations Topics Section Questions Covered in This Section Analyzing and Interpreting IP Addresses and Subnets 1–10 Scaling the IP Address Space for the Internet 11–14 0945_01f.book Page 313 Wednesday, July 2, 2003 3:53 PM 314 Chapter 12: IP Addressing and Subnetting 1. Which of the following is the result of a Boolean AND between IP address 150.150.4.100, mask 255.255.192.0? a. 1001 0110 1001 0110 0000 0100 0110 0100 b. 1001 0110 1001 0110 0000 0000 0000 0000 c. 1001 0110 1001 0110 0000 0100 0000 0000 d. 1001 0110 0000 0000 0000 0000 0000 0000 2. If mask 255.255.255.128 were used with a Class B network, how many subnets could exist, with how many hosts per subnet, respectively? a. 256 and 256 b. 254 and 254 c. 62 and 1022 d. 1022 and 62 e. 510 and 126 f. 126 and 510 3. If mask 255.255.255.240 were used with a Class C network, how many subnets could exist, with how many hosts per subnet, respectively? a. 16 and 16 b. 14 and 14 c. 12 and 12 d. 8 and 32 e. 32 and 8 f. 6 and 30 g. 30 and 6 CAUTION The goal of self-assessment is to gauge your mastery of the topics in this chapter. If you do not know the answer to a question or are only partially sure of the answer, you should mark this question wrong for purposes of the self-assessment. Giving yourself credit for an answer that you correctly guess skews your self-assessment results and might provide you with a false sense of security. 0945_01f.book Page 314 Wednesday, July 2, 2003 3:53 PM “Do I Know This Already?” Quiz 315 4. Which of the following IP addresses would not be in the same subnet as 190.4.80.80, mask 255.255.255.0? a. 190.4.80.1 b. 190.4.80.50 c. 190.4.80.100 d. 190.4.80.200 e. 190.4.90.1 f. 10.1.1.1 5. Which of the following IP addresses would not be in the same subnet as 190.4.80.80, mask 255.255.240.0? a. 190.4.80.1 b. 190.4.80.50 c. 190.4.80.100 d. 190.4.80.200 e. 190.4.90.1 f. 10.1.1.1 6. Which of the following IP addresses would not be in the same subnet as 190.4.80.80, mask 255.255.255.128? a. 190.4.80.1 b. 190.4.80.50 c. 190.4.80.100 d. 190.4.80.200 e. 190.4.90.1 f. 10.1.1.1 0945_01f.book Page 315 Wednesday, July 2, 2003 3:53 PM 316 Chapter 12: IP Addressing and Subnetting 7. Which of the following subnet masks would allow a Class B network to allow subnets to have up to 150 hosts and allow for up to 164 subnets? a. 255.0.0.0 b. 255.255.0.0 c. 255.255.255.0 d. 255.255.192.0 e. 255.255.240.0 f. 255.255.252.0 g. 255.255.255.192 h. 255.255.255.240 8. Which of the following subnet masks would allow a Class A network to allow subnets to have up to 150 hosts and would allow for up to 164 subnets? a. 255.0.0.0 b. 255.255.0.0 c. 255.255.255.0 d. 255.255.192.0 e. 255.255.240.0 f. 255.255.252.0 g. 255.255.255.192 h. 255.255.255.240 9. Which of the following are valid subnet numbers in network 180.1.0.0, when using mask 255.255.248.0? a. 180.1.2.0 b. 180.1.4.0 c. 180.1.8.0 d. 180.1.16.0 e. 180.1.32.0 f. 180.1.40.0 0945_01f.book Page 316 Wednesday, July 2, 2003 3:53 PM “Do I Know This Already?” Quiz 317 10. Which of the following are valid subnet numbers in network 180.1.0.0, when using mask 255.255.255.0? a. 180.1.2.0 b. 180.1.4.0 c. 180.1.8.0 d. 180.1.16.0 e. 180.1.32.0 f. 180.1.40.0 11. Which of the following best describes a feature of CIDR? a. Grouping a large number of Class C networks into a single group, and putting a single entry for that group in an Internet router, to reduce the overall size of the IP routing table b. To represent hundreds or thousands of client TCP or UDP connections from differ- ent hosts as that same number of connections, but making it appear as if all connec- tions are from one host c. The use network 10.0.0.0 in an Enterprise network d. The use of addresses such as 0000:0000:0000:0000:0000:FFFF:FFFF:0A01:0101 12. The phrase “to represent hundreds or thousands of client TCP or UDP connections from different hosts as that same number of connections, but making it appear as if all connections are from one host” best describes which of the following tools? a. Private addressing b. CIDR c. NAT d. IPv6 13. The phrase “grouping a large number of Class C networks into a single group, and putting a single entry for that group in an Internet router, to reduce the overall size of the IP routing table ” best describes which of the following tools? a. Private addressing b. CIDR c. NAT d. IPv6 0945_01f.book Page 317 Wednesday, July 2, 2003 3:53 PM 318 Chapter 12: IP Addressing and Subnetting 14. The phrase “the use network 10.0.0.0 in an enterprise network” best describes which of the following tools? a. Private addressing b. CIDR c. NAT d. IPv6 The answers to the “Do I Know This Already?” quiz are found in Appendix A, “Answers to the ‘Do I Know This Already?’ Quizzes and Q&A Sections.” The suggested choices for your next step are as follows: ■ 11 or less overall score—Read the entire chapter. This includes the “Foundation Topics” and “Foundation Summary” sections and the Q&A section. ■ 12, 13, or 14 overall score—If you want more review on these topics, skip to the “Foundation Summary” section and then go to the Q&A section. Otherwise, move to the next chapter. 0945_01f.book Page 318 Wednesday, July 2, 2003 3:53 PM IP Addressing Review 319 Foundation Topics This chapter begins with a brief review of IP addressing and subnetting. Following that, the text takes a thorough look at several types of IP addressing questions and the math you can use to find the answers. IP Addressing Review Chapter 5 explained the concepts behind IP addressing; Class A, B, and C networks; and subnetting. Before looking at the math behind IP addressing, a quick review will be helpful. Many different Class A, B, and C networks exist. Table 12-2 summarizes the possible network numbers, the total number of each type, and the number of hosts in each Class A, B, and C network. *The “Valid Network Numbers” row shows actual network numbers. There are several reserved cases. For example, networks 0.0.0.0 (originally defined for use as a broadcast address) and 127.0.0.0 (still available for use as the loopback address) are reserved. Networks 128.0.0.0, 191.255.0.0, 192.0.0.0, and 223.255.255.0 also are reserved. Without subnetting, a different IP network must be used for each physical network. For example, Figure 12-1 shows three example IP addresses, each from a different network. One address is in a Class A network, one is in a Class B network, and one is in a Class C network. Table 12-2 List of All Possible Valid Network Numbers* Class A Class B Class C First Octet Range 1 to 126 128 to 191 192 to 223 Valid Network Numbers 1.0.0.0 to 126.0.0.0 128.1.0.0 to 191.254.0.0 192.0.1.0 to 223.255.254.0 Number of Networks of This Class 2 7 – 2 2 14 – 2 2 21 – 2 Number of Hosts per Network 2 24 – 2 2 16 – 2 2 8 – 2 Size of Network Part of Address (bytes) 1 2 3 Size of Host Part of Address (bytes) 3 2 1 0945_01f.book Page 319 Wednesday, July 2, 2003 3:53 PM 320 Chapter 12: IP Addressing and Subnetting Figure 12-1 Example Class A, B, and C IP Addresses and Their Formats By definition, an IP address that begins with 8 in the first octet is in a Class A network, so the network part of the address is the first byte, or first octet. An address that begins with 130 is in a Class B network; by definition, Class B addresses have a 2-byte network part, as shown. Finally, any address that begins with 199 is in a Class C network, which has a 3-byte network part. Also by definition, a Class A address has a 1-byte host part, Class B has a 2- byte host part, and Class C has a 1-byte host part. Humans simply can remember the numbers in Table 12-2 and the concepts in Figure 12-1 and then quickly determine the network and host part of an IP address. Computers, however, use a mask to define the size of the network and host parts of an address. The logic behind the mask results in the same conventions of Class A, B, and C networks that you already know, but the computer can deal with it better as a binary math problem. The mask is a 32-bit binary number, usually written in dotted-decimal format. The purpose of the mask is to define the structure of an IP address. In short, the mask defines the size of the host parts of an IP address, representing the host part of the IP address with binary 0s in the mask. Class A mask has its last 24 bits as binary 0, which means that the last three octets of the mask are 0s. Table 12-3 summarizes the default masks and reflects the sizes of the two parts of an IP address. Table 12-3 Class A, B, and C Networks—Network and Host Parts and Default Masks Class of Address Size of Network Part of Address, in Bits Size of Host Part of Address, in Bits Default Mask for Each Class of Network A 8 24 255.0.0.0 B 16 16 255.255.0.0 C 24 8 255.255.255.0 0945_01f.book Page 320 Wednesday, July 2, 2003 3:53 PM IP Addressing Review 321 IP Subnetting IP subnetting creates vastly larger numbers of smaller groups of IP addresses, compared with simply using Class A, B, and C conventions. The Class A, B, and C rules still exist—but now a single Class A, B, or C network can be subdivided into many smaller groups. Subnetting treats a subdivision of a single Class A, B, or C network as if it were a network itself. By doing so, a single Class A, B, or C network can be subdivided into many nonoverlapping subnets. Figures 12-2 and 12-3 show the basic differences between a network that does not use subnetting and one that does use subneting. First, look at Figure 12-2, which uses six different IP networks. Figure 12-2 Network Topology Using Six IP Networks The design in Figure 12-2 requires six groups, each of which is a Class B network. The four LANs each use a single Class B network. In other words, the LANs attached to Routers A, B, C, and D are each a separate network. Additionally, the two serial interfaces composing 150.1.0.0 150.2.0.0 150.4.0.0 150.3.0.0 D C Hannah AB Frame Relay 150.5.0.0 Ray Fay Kris Wendell Vinnie Jessie 150.6.0.0 0945_01f.book Page 321 Wednesday, July 2, 2003 3:53 PM 322 Chapter 12: IP Addressing and Subnetting the point-to-point serial link between Routers C and D use the same network because these two interfaces are not separated by a router. Finally, the three router interfaces composing the Frame Relay network with Routers A, B, and C are not separated by an IP router and would compose the sixth network. As in Figure 12-2, the design in Figure 12-3 requires six groups. Unlike Figure 12-2, Figure 12-3 uses six subnets, each of which is a subnet of a single Class B network. Figure 12-3 Same Network Topology, Using One IP Network, with Six Subnets This design subnets Class B network 150.150.0.0. The IP network designer has chosen a mask of 255.255.255.0, the last octet of which implies 8 host bits. Because it is a Class B network, there are 16 network bits. Therefore, there are 8 subnet bits, which happen to be bits 17 through 24—in other words, the third octet. CAUTION Other Frame Relay IP addressing options would require one or two more IP network numbers for this physical network. 150.150.1.0 150.150.2.0 150.150.2.1 150.150.2.2 150.150.6.0 150.150.4.0 150.150.3.0 Hannah Frame Relay 150.150.5.0 Ray Fay Kris 150.150.4.2 Wendell Vinnie Jessie D C AB 0945_01f.book Page 322 Wednesday, July 2, 2003 3:53 PM [...]... last Tables 1 2-2 0 through 1 2-2 4 summarize the answers for the five examples used in this section Table 1 2-2 0 Subnet Chart—130 .4. 102.1/255.255.255.0 Octet 2 3 4 Address 130 4 102 1 Mask 255 255 255 0 Subnet number 130 4 102 0 First address 130 4 102 1 Broadcast 130 4 102 255 Last address 130 4 102 2 54 1 2 3 4 Address 130 4 102 1 Mask 255 255 252 0 Subnet number 130 4 100 0 First address 130 4 100 1 Broadcast... the process will be covered after Tables 1 2-3 2 and 1 2-3 3, which list two familiar examples, with the first two steps completed Table 1 2-3 2 Subnet List Chart—130 .4. 0.0/ 24 Octet 2 3 4 Network number 130 4 0 0 Mask 255 255 255 0 Subnet zero Table 1 2-3 3 1 130 4 0 0 Subnet List Chart—130 .4. 0.0/22 Octet 1 2 3 4 Network number 130 4 0 0 Mask 255 255 252 0 Subnet zero 130 4 0 0 The last step in this process, Step... Broadcast 130 4 103 255 Last address Table 1 2-2 1 1 130 4 103 2 54 Subnet Chart—130 .4. 102.1/255.255.252.0 Octet Table 1 2-2 2 Subnet Chart—8.1 .4. 5/255.255.0.0 Octet 1 2 3 4 Address 8 1 4 5 255 255 0 0 Subnet number 8 1 0 0 First address 8 1 0 1 Broadcast 8 1 255 255 Last address 8 1 255 2 54 Mask 0 945 _01f.book Page 338 Wednesday, July 2, 2003 3:53 PM 338 Chapter 12: IP Addressing and Subnetting Table 1 2-2 3 Subnet... are indeed always reserved Table 1 2-9 summarizes the five examples used so far in this chapter Table 1 2-9 Five Example Addresses/Masks, with Number of Network, Subnet, and Host Bits Address 8.1 .4. 5/16 130 .4. 102.1/ 24 199.1.1.100/ 24 130 .4. 102.1/22 199.1.1.100/27 Mask 255.255.0.0 255.255.255.0 255.255.255.0 255.255.252.0 255.255.255.2 24 Number of network bits 8 16 24 16 24 Number of host bits 16 8 8 10... memorize Table 1 2-3 0 Table 1 2-3 0 Maximum Number of Subnets/Hosts Number of Bits in the Host or Subnet Field Maximum Number of Hosts or Subnets (2x – 2) 1 0 2 2 3 6 4 14 5 30 6 62 7 126 8 2 54 9 510 10 1022 11 2 046 12 40 94 13 8190 14 16,382 As you can see, if you already have the powers of 2 memorized, you really do not need to memorize the table—just remember the formula As for the first example question,... number The following tables (Tables 1 2-1 0 through 1 2-1 4) show the Boolean AND process for the five examples used in the previous section of this chapter: Table 1 2-1 0 Boolean AND Calculation for Subnet, Address 8.1 .4. 5, Mask 255.255.255.0 Address 0000 1000 0000 0001 0000 0100 0000 0101 Mask 255.255 0.0 1111 1111 1111 1111 0000 0000 0000 0000 AND result Table 1 2-1 1 8.1 .4. 5 8.1.0.0 0000 1000 0000 0001 0000... host part of the address The size of the network part of the address never shrinks—in other words, Class A, B, and C rules still apply when defining the size of the network part of an address However, the host part of the address shrinks to make room for the subnet part of the address Figure 1 2 -4 shows the format of addresses when subnetting Figure 1 2 -4 Address Formats When Subnetting Is Used 8 24 –... the same example, but with the subnet number and first valid IP address shown Table 1 2-2 8 Subnet Chart—130 .4. 102.1/255.255.252.0, with Subnet and First IP Address Octet 1 2 3 4 Comments Address 130 4 102 1 Mask 255 255 252 0 Subnet number 130 4 100 0 Magic = 256 – 252 = 4; 4 ∗ 25 = 100, closest multiple < 102 First address 130 4 100 1 Add 1 to subnet’s last octet Broadcast 130 4 Last address 130 4 To review,... mask You also can find 25 more problems with solutions 0 945 _01f.book Page 340 Wednesday, July 2, 2003 3:53 PM 340 Chapter 12: IP Addressing and Subnetting on the CD chapter titled, “Subnetting Practice: 25 Subnetting Questions.” The process uses something I call a subnet chart, as shown in Table 1 2-2 5 Table 1 2-2 5 Subnet Chart—Generic Octet 1 2 3 4 Address Mask Subnet number First address Broadcast Last... AND Calculation for Subnet, Address 130.1.102 .4, Mask 255.255.255.0 Address 130 .4. 102.1 1000 0010 0000 0100 0110 0110 0000 0001 Mask 255.255.255.0 1111 1111 1111 1111 1111 1111 0000 0000 AND result 130 .4. 102.0 1000 0010 0000 0100 0110 0110 0000 0000 0 945 _01f.book Page 3 34 Wednesday, July 2, 2003 3:53 PM 3 34 Chapter 12: IP Addressing and Subnetting Table 1 2-1 2 Boolean AND Calculation for Subnet, Address . 190 .4. 80.80, mask 255.255.255 .128 ? a. 190 .4. 80.1 b. 190 .4. 80.50 c. 190 .4. 80.100 d. 190 .4. 80.200 e. 190 .4. 90.1 f. 10.1.1.1 0 945 _01f.book Page 315 Wednesday, July 2, 2003 3:53 PM 316 Chapter 12: . As in Figure 1 2-2 , the design in Figure 1 2-3 requires six groups. Unlike Figure 1 2-2 , Figure 1 2-3 uses six subnets, each of which is a subnet of a single Class B network. Figure 1 2-3 Same Network. network part of an address. However, the host part of the address shrinks to make room for the subnet part of the address. Figure 1 2 -4 shows the format of addresses when subnetting. Figure 1 2 -4 Address