Appendix N

Classless Inter-domain Routing

The chapters in Part IV of the book work through the fundamentals of IPv4 addressing. However, most of the discussion about starting with a big block of addresses, and then cre- ating smaller blocks called subnets, begins with a classful network (that is, a Class A, B, or C network). A thorough understanding of how to take a classful network and subdivide it into subnets is very important, because most enterprises do exactly that: They start with some private IPv4 network, often network 10.0.0.0, and divide it into subnets. However, most enterprises also use Classless Inter-domain Routing (CIDR) with regard to the public IPv4 addresses used by the company. CIDR defines many ideas, including how an enterprise can be assigned a block of public IPv4 addresses called a classless prefix. Like Class A, B, or C networks, a CIDR classless prefix is a block of consecutive IP addresses. However, CIDR classless prefixes can be a variety of sizes—any power of 2—rather than the three sizes of classful networks based on Class A, B, and C rules. This appendix uses three major sections to introduce the topics. The first discusses the meaning and use of CIDR when a classless prefix is assigned to an enterprise. The second major section then examines subnetting of a classless prefix inside an enterprise. The chap- ter ends with a section about terminology and calculations to help you connect the terms and math you learned in Part IV with the details covered in this appendix.

Foundation Topics Using CIDR Classless Prefixes Originally, classful IP networks played a big role in the design of the global Internet. The Internet relied on the idea that every computer would use a globally unique IPv4 address. To achieve that goal, the early administrators for the Internet created an administrative process. Each company, government branch, or other organization would be assigned one public IP network (a Class A, B, or C network). Only that company could use that particular Class A, B, or C network, preventing overlapping IP addresses between two companies. The one-classful-network-per-company strategy worked well as long as there were enough classful networks. However, the Internet grew, and it reached a stage where it grew very quickly, to the point that it was clear that the world would run out of IPv4 addresses. The long-term solution is to migrate from IPv4 to IPv6, with a 128-bit address. At the same time, the IETF came up with two other very useful tools to make better use of the existing IPv4 address space: NAT (as discussed in Chapter 27, “Network Address Translation”) and CIDR (as discussed in this appendix). 2 CCENT/CCNA ICND1 100-105 Official Cert Guide

That original strategy of assigning each company an entire classful IP network worked well to ensure unique addresses, but the three-sizes-fits-all approach happened to waste address- es. That is, a company would receive an address block of the size of either a Class A, Class B, or Class C of size 224, 216, and 28, respectively. Sometimes those sizes matched the true needs of the company, but often those sizes did not. CIDR allows blocks of consecutive addresses—called classless prefixes—that come in sizes of powers of 2. Basically, CIDR lets us continue to assign IPv4 address so that they are unique but better match the size of the address block to the needs of each company. This first major section of this appendix discusses the ideas surrounding how the number- ing authorities assign a public CIDR classless prefix to an enterprise. First, this section explains the administrative process to assign public IP address blocks. Second, this section shows why the old methods wasted too many IPv4 addresses and why CIDR wastes fewer addresses. Then, once you know the terms and ideas, this section reviews the math that lets you better understand what a company receives when it is assigned a classless prefix. Note that the math works much like the math from Parts IV and VI in the book. The Public IPv4 Address Assignment Process and Players Originally, way back in the history of IP, the Internet Assigned Numbers Authority (IANA) performed all public address assignments. Each enterprise contacted IANA and applied for a block of public addresses. IANA then considered the request. Once approved, IANA provided some documentation. And in those days, the assigned address block was some Class A, B, or C network, with that public network number now assigned to that company. Simple enough. You can still see some of those original assignments of Class A networks listed on the IANA.org website. Just go to the site and find the IP Address Allocations link, and you will find the list. Figure N-1 shows a few of those Class A networks for perspective.

General Electric AT&T Bell Labs 3.0.0.0/8 12.0.0.0/8

IBM The HP 9.0.0.0/8 Internet 15.0.0.0/8

US DoD Apple Computer 11.0.0.0/8 17.0.0.0/8

Figure N-1 Example Class A Assignments from IANA As the Internet grew in popularity, IANA changed the address assignment process, in part due to growth and in part to better support the global nature of the process. Rather than having all requests come to IANA (whose offices were and still are in the United States), IANA distributed the address assignment work around the world to five different regional organizations called Regional Internet Registries (RIR). IANA ultimately owns all the IPv4 Appendix N: Classless Inter-domain Routing 3

address spaces worldwide, but the RIRs ask IANA to allocate address blocks to the RIR. The RIR in turn assigns blocks of addresses to enterprises for them to use when connect- ing to the Internet (or further allocates addresses to Internet service providers [ISP], which in turn assign the addresses for use by their customers). Figure N-2 shows the names of the RIRs and the general flow of assignments of public IP addresses.

IANA

ARIN LACNIC RIPE AFRINIC APNIC

ISP ISP ISP ISP ISP

N

Enterprise Enterprise Enterprise Enterprise Enterprise

Figure N-2 IANA, RIRs, ISPs, and Organizations That Use IPv4 Addresses The figure shows the process as it normally works today. For example, ISPs can (and typi- cally do) receive the allocation of an address block from one of the RIRs. Then, the ISP can assign subsets of its address block to its customers. In the end, each enterprise can choose to apply for a block of globally unique IPv4 addresses to use; it can apply to its RIR or to any ISP that serves its geography. The Need for More Granular Block Size Assignment The process shown in Figure N-2 helped to distribute the administrative effort to assign IPv4 addresses worldwide. However, the process itself did nothing to prevent the waste that occurred by assigning only Class A, B, and C address blocks. So, how did the process of using only three sizes of public IP address blocks cause waste? To understand, you need to connect a couple of points. First, recall the size of Class A, B, or C networks, as listed here. Then notice how huge the difference is between the number of addresses in each. Class A: 16,777,216 Class B: 65,536 Class C: 256

Second, focus on the fact that public addresses assigned to one company cannot be used by another. Suppose, for example, that an enterprise is assigned some Class B network. It uses 10,000 or so IPv4 addresses. What about the other 55,000 or so in that Class B network? Wasted. No other company can use them. Or imagine a company has a Class A network and uses even 1 million addresses (that would be a pretty large company). How much waste? About 15.7 million addresses. 4 CCENT/CCNA ICND1 100-105 Official Cert Guide

NOTE Here is an analogy that might make the two main points more memorable. Imagine that the grocery store sold bread in loaves of two sizes: 2 slices or 1000 slices. You need more than two slices of bread, but for some reason, you are only allowed to buy one loaf— either the one with 2 slices or the loaf with 1000 slices. And they cost about the same. What do you do? You might just buy the loaf with 1000 slices, knowing that most of that bread will go to waste.

CIDR attacked the problem of wasted public IP addresses with several interrelated features. Most importantly for this appendix, CIDR allowed the assignment of address blocks of any power of 2. In comparison, think of Class A, B, and C networks as address blocks of size 224, 216, and 28, respectively. CIDR defines rules for any useful power of 2. For example:

■ A company needed 1000 addresses and received an entire Class B network, wasting 64,000+ addresses. With CIDR, the company received an address block with 1024 (210) addresses, with little waste. ■ A company needed 10,000 addresses and received an entire Class B address, wasting 55,000+ addresses. With CIDR, the company received an address block with 16,384 (214) addresses, wasting about 6,000 addresses. ■ A company needed 50 addresses and received an entire Class C network, wasting about 200 addresses. With CIDR, the company received an address block with 64 (26) address- es, wasting about 14 addresses.

CIDR Address Assignment These next few pages take material you should know well by now—how a Class A, B, and C network works—and contrast that with the classless prefixes defined by CIDR. First, the text reviews some key points about how public classful networks can be assigned, followed by a contrasting view of classless prefix assignment.

NOTE CIDR does define the details of how to assign public IP address blocks as any power of 2, but CIDR also defines other associated conventions and ideas. This appendix discusses only the address assignment concepts from CIDR. For the curious, feel free to read more in RFC 4632, “Classless Inter-Domain Routing (CIDR): An Address Assignment and Aggregation Strategy.”

A Brief Review of Public Classful Network Assignment Think back to address assignment before CIDR. A company applies for and is assigned a public classful IPv4 network. The document from the RIR or ISP, as given to the enterprise, includes these key facts:

■ The specific classful network ID (that is, the Class A, B, or C network ID) ■ The prefix length—either explicitly listed in the document or implied as the default mask—used with that class of network (specifically, prefix lengths /8, /16, or /24, for Classes A, B, and C, respectively). Appendix N: Classless Inter-domain Routing 5

Based on those two facts, you can find other key facts about that network, as follows:

■ 32 minus the prefix length tells you the number of host bits in the unsubnetted network (specifically, 24 host bits in a Class A network, 16 host bits in a Class B network, and 8 in a Class C network). ■ Interestingly, that classful network ID is the first/lowest dotted-decimal number in the range of public IPv4 addresses in that network. ■ The network ID and prefix length gives you enough information to calculate the num- bers in the address range (as shown in detail in Chapter 14, “Analyzing Classful IPv4 Networks”).

Figure N-3 summarizes the idea of the prefix length for Classes A, B, and C, and the result- ing size of the host parts of those networks.

Network (8) Host (24) Size: 224 – 2 N

Network (16) Host (16) Size: 216 – 2

Network (24) Host (8) Size: 28 – 2

Figure N-3 Structure of Unsubnetted Class A, B, and C Networks For example, a company might be assigned classful network 9.0.0.0/8, meaning Class A net- work 9.0.0.0 with default mask /8 (255.0.0.0). Following the items in the bulleted list:

■ With a prefix length of /8, the network part of the address is length 8, leaving 24 host bits, so the Class A network’s size is 224 addresses, ignoring reserved values. ■ 9.0.0.0 (the network ID) is also the first/lowest number in the range of numbers in Class A network 9.0.0.0 (although it is a reserved value).

Finally, what are the 224 addresses in network 9.0.0.0? Chapter 14 works through that math in some detail. However, for a quick review, the following is the range of addresses:

■ The numerically lowest number is the network ID. ■ The numerically highest number is the network broadcast address.

Continuing that same example with Class A network 9.0.0.0, mask 255.0.0.0, Figure N-4 shows the range of addresses in Class A network 9.0.0.0. (Note that although the network ID and network broadcast address are the literal lowest and highest number in the range, they cannot be assigned to a host for use as an IP address.) 6 CCENT/CCNA ICND1 100-105 Official Cert Guide

Network ID 9 . 0 . 0 . 0 First Address 9 . 0 . 0 . 1 Last Address 9.255 .255 .254 Network Broadcast Address 9.255 .255 .255

Figure N-4 Range of Addresses in Class A Network 9.0.0.0 Public CIDR Block Assignment Before CIDR, the numbering authority assigned an enterprise a classful network number and associated (default) mask, from which you calculated the size of the address block and the range of addresses in the block. CIDR uses the same general ideas with the assignment of a classless prefix:

■ The documentation received from the RIR or ISP lists a CIDR classless prefix. This class- less prefix is a dotted-decimal number (DDN), and it serves the same purpose as a net- work ID. It is also the first/lowest DDN in the address block. ■ The document includes the classless prefix length (mask) for the block of addresses. ■ You can then calculate the number of host bits in the unsubnetted classless prefix, the size (number of addresses) in the block, and the range of addresses in the block.

Figure N-5 summarizes the terms for what a company receives before CIDR and with CIDR address assignment.

Pre-CIDR CIDR

Classful Classless Network ID Prefix ID

Default Mask Classless (/8, /16, /24) Prefix Length

Sizes: 28, 216, 224 Sizes: 2x

Figure N-5 Address Assignment Terms Before and with CIDR When using classless prefixes, you must quit thinking about class rules. Just do the math based on the input received from the RIR or ISP. In fact, that’s what the classless in classless inter-domain routing refers to: Quit using classful rules. Instead, ignore Classes A, B, and C, use the CIDR classless prefix ID (the first number in the range) and the CIDR classless prefix length, and do the math to figure out the details like the number of addresses and the range of addresses. For example, consider a case in which a company has received a classless prefix document- ed as follows: Classless prefix ID: 128.66.4.0 Classless prefix length: /22 Appendix N: Classless Inter-domain Routing 7

First, before the company subnets this classless prefix into smaller subnets, this block of addresses exists as a single block. How many addresses are in this classless prefix? Well, similar to classful networks, an unsubnetted classless prefix also has a 32-bit two-part struc- ture. The classless prefix length, as listed in the documentation from the RIR or ISP, defines the length of the first part. The second part is the host part, and the two parts add up to 32. Figure N-6 shows an unsubnetted classless prefix length of /22 (as is used with this most recent example).

Classless Prefix Length Host

22 10

Addresses: 210

Figure N-6 Finding the Size of a /22 CIDR Classless Prefix The size of a CIDR classless prefix is 2H; that is, 2 to the number of host bits, without sub- N tracting 2. However, the fact that we do not subtract 2 in this case is trivia. But just so you know a bit of background:

■ The original definition of classful networks reserved the network number and network broadcast address, so we subtract 2 when calculating the number of addresses per class- ful network. ■ CIDR does not literally reserve the first and last number in the classless prefix, so we do not subtract 2.

NOTE In reality, that small difference does not actually give you more addresses to use. Once a company takes that classless prefix and performs subnetting, the first and last num- bers in each subnet end up being reserved as either a subnet ID or subnet broadcast address anyway. There is just no formal reservation of the first and last address in the assigned class- less prefix.

The other bit of math to do with an assigned classless prefix is to find the range of address- es in the address block. With Class A, B, and C networks, you could use some pretty basic rules to find the range (as shown in detail in Chapter 14). With classless prefixes, you must go to a little more effort—but you already know the math! The math to find the range of addresses in a classless prefix is identical to the math used to find the range of addresses in one subnet. If you got somewhat comfortable with the process to find the range of addresses in a subnet (in Chapter 16, “Analyzing Existing Subnets”), great, you should be ready to do the same math with classless prefixes. Basically, you take the two key pieces of information from the classless prefix assignment— the classless prefix ID and the prefix length—and treat them like the subnet ID and the subnet mask, respectively. Then do the same math process shown in Chapter 16 to find the range of IP addresses. Figure N-7 picks up the example of classless prefix 128.66.4.0/22 and shows part of the calculation to find the range of addresses. The figure begins with the lowest number in the 8 CCENT/CCNA ICND1 100-105 Official Cert Guide

range (128.66.4.0) already listed. The figure shows the decimal process shown in Chapter 14, which is as follows: 1. The DDN version of the /22 mask as 255.255.252.0 2. The calculated magic number (256 – 252 = 4) 3. The per-octet logic to convert the number at the low end of the range (128.66.4.0) to the number at the high end of the range (128.66.7.255)

256 –252 0-255 255 . 255 . 252 . 0 4

ID 128 . 66 . 4 . 0

Action Copy Copy +Magic 255 –1 128 . 66 . 7 . 255

Figure n-7 Calculating the End of the Address Range for 128.66.4.0/22 That completes the descriptions of why and how classless prefixes are given to individual enterprises. The rest of the first major section of this appendix gives you a chance to prac- tice the math. Practice Analyzing Classless Prefixes Imagine that an exam question clearly tells you that a company uses a classless prefix. The question identifies the classless prefix ID with a DDN value, along with the prefix length. From that information, you should be ready to do these tasks:

■ Determine the size of the block (that is, the number of addresses in the block) ■ Determine the first and last numbers in that range of addresses in that address block

Earlier, Figures N-6 and N-7 and the surrounding text showed an example of how to find these pieces of information. This section summarizes the math processes and gives you some practice questions. First, the formal process to find the size of a classless prefix is as follows: Step 1. Find the prefix length (X) of the classless prefix as assigned to the company (from documentation). Step 2. Calculate the number of host bits as H = 32 – X. Step 3. The size of the block is 2H (ignoring reserved values).

The logic is much more obvious from a figure like Figure N-8 as well. Appendix N: Classless Inter-domain Routing 9

32 X 32 - X

Classless Prefix Length Host (H)

Figure N-8 Logic Behind Calculation of the Size of a Classless Prefix To find the range of addresses in the classless prefix, you just have to repeat the same sub- netting math you learned in Chapter 16. To find the range of addresses in the CIDR block, use that same math, starting with the CIDR classless prefix ID and the assigned CIDR prefix length. Summarizing the steps: Step 1. Treat the assigned CIDR classless prefix as the subnet ID. Step 2. Treat the assigned CIDR prefix length as the subnet mask. Step 3. Use any method that you normally use to calculate the subnet ID and subnet broadcast address with these two values to find the lowest and highest num- N bers in the classless prefix range.

Now it is time to practice. Table N-1 shows several classless prefix assignments. First, calculate the size of the address block. Second, calculate the last number in the range of addresses. Note that the first number in the range is the classless prefix ID, so no calculation is needed.

NOTE Refer to Appendix A, “Numeric Reference Tables,” if you need a list of powers of 2.

Table N-1 CIDR Block Practice: Size of Block and Range of Numbers Problem Classless Prefix Size Last Number in Range Prefix ID Length 1 128.66.32.0 /21 2 128.66.64.0 /23 3 192.51.100.0 /22 4 192.51.96.0 /20 5 128.66.1.128 /26 6 128.66.3.96 /28 7 192.51.100.192 /29 8 192.51.100.64 /27

You can find the answers to these practice problems at the end of this appendix, in the sec- tion titled “Answers to Earlier Practice Problems.” 10 CCENT/CCNA ICND1 100-105 Official Cert Guide

Understanding Subnetted CIDR Classless Prefixes When a company asks for and receives the right to use a public classful IP network, or a public classless prefix, the company receives the right to use all the addresses in that one address block. But that is just part of the story. Once received, that company’s networking team takes that one block of addresses and subdivides it into smaller groups of addresses for use on various LANs and WANs in the company network. In other words, the engineers still need to create subnets, whether starting with a classful prefix or a classful network. Part IV of the book already set the stage for many of the big ideas in this chapter, but those chapters assumed a starting point of a single classful network. These next few pages now add CIDR classless prefixes into the mix, but using the same big ideas you already learned back in Part IV of the book. In particular, think about what you have already learned about existing subnet designs. Back in Chapter 15, “Analyzing Subnet Masks,” the storyline went something like this:

■ Someone else, before today, either received a public IP network from a numbering authority or chose a private IP network to use for the company. The point is that the starting point was an entire classful network. ■ Someone else, before today, made a design choice to use one mask, and one mask only, for all subnets. ■ Someone else, before today, chose a subnetting plan for the company, specifically choosing that one mask to use for all subnets.

Still thinking back to Chapter 15, that chapter discusses how to interpret what those other people created when they made the choices in that list. And that analysis required that we think about the class of network used as a starting point. Now think about using a classless prefix as a starting point, instead of either a public or a private classful network. The same kinds of things happen. Someone else, before today, asked for and obtained the classless prefix. Someone else already made the design choice to use one subnet mask, and they already chose that mask’s value. For the next few pages, the text reviews the process of analyzing those existing subnetting choices, first when using a classful network and then when using a classless prefix. Review: Interpreting Subnet Design That Starts with a Network Chapter 15’s section titled “Identifying Subnet Design Choices Using Masks”—basically, the second half of that chapter—discusses how to interpret someone else’s subnet design. That discussion assumes the following:

■ A starting point of a classful network ■ A single subnet mask used for all subnets

With those assumptions, you can use simple addition and subtraction to find the structure of the subnetting design. You should be given the network number that is subnetted, from which you can derive the class, and remember the size of the network part of the subnet design. You should be given the one subnet mask used. From there, you can easily calculate the size of the subnet and host parts of the design using the details shown in Figure N-9. Appendix N: Classless Inter-domain Routing 11

32 /P

N S H

Class: A: N = 8 B: N = 16 C: N = 24

Figure N-9 Structure of a Subnetting Design, Starting with a Classful Network In case the variables in the figure are not obvious: P: The prefix length of the subnet mask per the subnetting design (given) N: The size of the network part (per classful rules) S: The size of the subnet field in the subnet design (calculated) N H: The size of the host field in the subnet design (calculated)

Once you calculate the size of the subnet and host parts of the subnetting design, you can find the number of subnets and hosts/subnet with these familiar formulas:

■ Subnets: 2S ■ Hosts/subnet: 2H – 2

For example, imagine you see an exam question with a design that uses the following:

■ A public Class B network 128.66.0.0 ■ A single subnet mask for all subnets inside this company: /24

Using these facts, you can analyze the design to determine the following: N = 16, because 128.66.0.0 is a Class B network P = 24, as given to you S = 8, because N + S = P H = 8, because H = 32 – P The number of subnets is 256 (28) The number of hosts/subnets is 254 (28 – 2)

Figure N-10 summarizes those same facts with the familiar structure diagram.

32 /P = 24

N = 16 S = 8 H = 8

Figure N-10 Structure of a Subnetting Design for Network 128.66.0.0, Prefix Length /24 12 CCENT/CCNA ICND1 100-105 Official Cert Guide

Assuming you have read and practiced the topics in Chapter 15, the above should indeed be a review. If these ideas do not sound familiar, you might want to go back to Chapter 15 for some review and practice. Interpreting a Subnet Design That Starts with a CIDR Classless Prefix When the company uses a CIDR classless prefix rather than a classful network, you follow the same kind of logic but with one difference. With CIDR, you ignore classful networking rules, so there is no network part, and no rule that says the network part is either 8, 16, or 24 bits. So what do you use instead? The short answer: Use the CIDR prefix length—that is, the prefix length as assigned to the company by the RIR or ISP—in the same way you use the length of network part of the address when starting with a classful network. The rest of the process works the same as before. For example, imagine you see an exam question with a design that uses the following:

■ A classless prefix of 128.66.32.0 /20 ■ A single subnet mask is used for all subnets inside the company: /24

Next, stop to think about this classless prefix before the company’s network engineer subnets the address block. The classless prefix structure looks like Figure N-11, with a /20 block length and a 12-bit host field. Basically, it is one block of 212 addresses, as assigned by an RIR or ISP.

32

/X = 20

X = 20 H = 12 No Subnetting! 1 Block, Size 212

Figure N-11 Structure of the /20 CIDR Block, Before Subnetting To create subnets, the network engineer who came before us chose a subnet mask with more prefix bits than the classless prefix assigned by the RIR or ISP (/20 in this case). In this example, the engineer chose a /24 subnet mask for all subnets, 4 bits longer than the class- less prefix length’s /20 mask. That design choice created a 4-bit subnet field, as shown in Figure N-12. It also left an 8-bit host field.

32 /P = 24 /X = 20

X = 20 S = 4 H = 8

24 Subnets 28 - 2 Hosts/Subnet

Figure N-12 Structure of the /20 CIDR Block, After Subnetting with /24 Appendix N: Classless Inter-domain Routing 13

Summarizing, you can analyze the subnet design with similar logic, whether starting with a classful network or a classless prefix. The one big difference: With classless prefixes, you cannot start with class rules and a network part of 8, 16, or 24 bits. Thankfully, the replace- ment logic is easy: Just use the classless prefix length as assigned to the company by the RIR or ISP, and then proceed with the same simple addition and subtraction. Practice Interpreting a Subnet Plan That Starts with a CIDR Block Once you know that a question requires you to analyze a subnet design, and that question uses a classless prefix, you should have a good idea of how to proceed. This next topic sum- marizes the rules (for easier review and study) and gives you some practice problems. First, the key points of the analysis, both when starting from a classful network or a classless prefix, are included in the following list: Step 1. Use P to represent the subnet mask’s value in prefix form (/P). (P does not rep- resent the classless prefix length assigned by the RIR or ISP.) Step 2. If subnetting a classful network, set aside N bits, value 8, 16, or 24, based on N the class of the network. Step 3. If subnetting a classless prefix, set aside X bits on the left equal to the classless prefix length as assigned to the company by the RIR or ISP. Step 4. Calculate the size of the subnet field (S), as P – N (when starting from a classful network), or P – X (when starting from a classless prefix). Step 5. Calculate the size of the host field (H) as 32 – P. Step 6. The size of the subnet and host parts of the design tell us how many subnets exist (2S) and how many hosts exist in each subnet (2H – 2).

Figure N-13 summarizes those ideas and variables for the case of using CIDR.

32 /P /X

X S H

X = Classless Prefix Length P = Enterprise Subnet Mask (Prefix Length)

Figure N-13 Breaking Down the Subnet Design with CIDR Try some practice problems to get used to the process. Treat each line in Table N-2 as a separate practice problem. In each case, the table already supplies the classless prefix length (variable X in the figure) and the subnet mask used for all subnets in the subnet design (vari- able P). Use that information to calculate the number of subnet and host bits, the resulting number of subnets and hosts/subnets. Refer to Appendix A for a list of the powers of 2. 14 CCENT/CCNA ICND1 100-105 Official Cert Guide

Table N-2 CIDR Block Practice: Size of Block and Range of Numbers CIDR Block CIDR Subnet S (Subnet H (Host # Subnets # Hosts ID Block Mask Bits) Bits) Length 1 128.66.32.0 /21 /24 2 128.66.64.0 /23 /26 3 192.51.100.0 /22 /28 4 192.51.96.0 /20 /25 5 128.66.1.128 /18 /23 6 128.66.3.96 /19 /25 7 192.51.100.192 /24 /26 8 192.51.100.64 /27 /29

You can find the answers to these practice problems at the end of this appendix, in the sec- tion titled “Answers to Earlier Practice Problems.” Challenges with CIDR Terminology and Processes This final of three major sections basically catches two topics that need special attention for the exam. First, this section briefly discusses how to calculate facts about existing subnets when an enterprise happens to have started with a classless prefix instead of a classful net- work. The final topic gives some perspective about the terminology surrounding CIDR, IP addressing, and subnetting. Calculating Subnet Facts When Using CIDR This short topic exists for those of you who wonder about this question: Does all this CIDR stuff change how I calculate the subnet ID, subnet broadcast address, and range of addresses in a subnet? The answer: Nothing changes at all. If you see a host’s IP address and subnet mask—the address and mask that the host uses— you already know (from Chapter 16) how to calculate the subnet ID, subnet broadcast address, and range of usable addresses in that subnet. Whether than enterprise happened to be using an entire classful private IP network, classful public IP network, or a classless pre- fix, those calculations about that one subnet remain the same. Period. CIDR Terminology If all exam questions, as well as all networkers, used one consistent formal term for every idea, then life would be much simpler. For instance, this book consistently uses the terms on the left of Figure N-14. Those three terms refer to the three categories of address blocks that an enterprise uses to build its IPv4 networks: either an entire private IP network based on RFC 1918 (for instance, network 10.0.0.0/8), a (classful) public IP network, or a CIDR classless prefix. Appendix N: Classless Inter-domain Routing 15

Choose Private Network

Address Assign Request Block Subnet Public Network Used By Design Company Assign Request Classless Prefix

Use This Mask: Mask Choices: Default Mask Any Subnet Masks or per Subnet Design Classless Prefix Length

Figure N-14 Formal Terms for Key IPv4 Address Blocks Used in This Book N The figure also shows the process flow for what happens with IPv4 addressing inside a company beginning with the original IPv4 addressing plan. Some network engineer chooses a private IP network to use, or receives an assignment of a public IP network or CIDR class- less prefix from an RIR or ISP. Then some engineer develops a subnetting plan on paper that tells the company how they will break down that one larger address block into subnets, which are used on individual LAN and WAN links. Unfortunately, the world does not use a single term for every idea, including the ideas sum- marized in Figure N-14. As a result, you must be ready to determine whether someone is asking about a private IP network, a public IP network, a CIDR classless prefix, or a subnet, or even if the distinction matters. First, Table N-3 summarizes the commonly used terms for the three major categories of address blocks.

Table N-3 CIDR Block Practice: Size of Block and Range of Numbers Formal Term Less-Ambiguous Alternatives More-Ambiguous Alternatives Private IP network Private network IP network, classful network, network, subnet, address block Public IP network Public network IP network, classful network, network, subnet, address block CIDR classless Classless prefix, CIDR prefix, Prefix, address block, block, prefix CIDR address block, CIDR block network, subnet

The problem comes when someone uses one of the more common but more ambiguous terms, like network. For example, compare these three phrases from sample exam questions: Company 1 uses network 172.16.0.0/16 with a subnetting plan… Company 1 uses network 128.66.0.0/16 with a subnetting plan… Company 1 uses network 128.66.4.0/22 with a subnetting plan… 16 CCENT/CCNA ICND1 100-105 Official Cert Guide

To interpret phrases such as these, with ambiguous words, focus on the numbers instead of the words. Written just above each other, you can see that each phrase uses the word network, but each phrase refers to a different type of address block: a private IP network in the first item, a public IP network in the second, and a CIDR classless prefix in the third. How do you know? You must look at the numbers, not the words. For instance, 172.16.0.0 is within the RFC 1918 range for private addresses and is one of the reserved private Class B networks, and /16 is the default mask for Class B networks. Clearly that example uses a private IP network. The last item uses a mask of /22, so it can- not be either a private or public IP network because /22 is not a default classful mask. As another example of ambiguous terms, the term address block has been used more and more to refer to all three categories (private IP networks, public IP networks, and CIDR classless prefixes). Why? Well, each is a set or block of consecutive addresses, so the term address block makes sense to a lot of people. Similarly, people often use the word subnet to refer to any set or block of addresses, whether an entire classful network, the CIDR classless prefix assigned by an RIR, or a true subnet as created by an enterprise subnetting plan. By now you know the bigger issues of dealing with CIDR and other IPv4 addressing termi- nology. The following list summarizes the steps you should take to be ready for this kind of terminology:

■ Learn the formal and unambiguous alternative terms from Table N-3. ■ When the words appear to be ambiguous, look at the numbers, and ask yourself the following: ■ Is it a private network per RFC 1918? ■ If it is from the public address range, is it using a default mask for a Class A, B, or C network? If so, it might be a public IP network. ■ If it is from the public address range, is it using a mask other than a Class A, B, or C network? If so, it is likely a CIDR classless prefix. ■ Be ready for these common terms that refer to any of the three categories (private, pub- lic, and CIDR block): network, subnet, prefix, and address block. ■ If the discussion begins with a host’s IP address/mask or a router’s IP address/mask, then the type of address block (private, public network, CIDR prefix) probably does not mat- ter. In these cases, you probably care most about the subnet ID and the range of address- es in the subnet, and you can find that from the IP address/mask.

Chapter Review

One key to doing well on the exams is to perform repetitive spaced review sessions. Review this chapter’s material using either the tools in the book, DVD, or interactive tools for the same material found on the book’s companion website. Refer to the “Your Study Plan” ele- ment for more details. Table N-4 outlines the key review elements and where you can find them. To better track your study progress, record when you completed these activities in the second column. Appendix N: Classless Inter-domain Routing 17

Table N-4 Chapter Review Tracking Review Element Review Date(s) Resource Used Review key topics Book Review key terms DVD/website

Review All the Key Topics Table N-5 Key Topics for Appendix N Key Topic Description Page Element Number Figure N-5 Comparison of public IP network assignment versus CIDR 6 classless prefix List Calculations for the size of a CIDR classless prefix 8 List Calculations for the range of addresses in a CIDR classless prefix 9 N Figure N-13 Structure of fields and relationships for a subnetted CIDR classless 13 prefix Table N-3 CIDR terms 15 List Advice on thinking through CIDR terms for exam questions 16

Key Terms You Should Know classless inter-domain routing, classless prefix, classless prefix length, address block, IANA, RIR, ISP

Answers to Earlier Practice Problems

This chapter includes practice problems spread around different locations in the chapter. Table N-6 lists the answers to questions posed earlier in Table N-1.

Table N-6 Answers to CIDR Block Practice from Table N-1 Problem CIDR Block ID Prefix Length Size Last Number in Range 1 128.66.32.0 /21 211 128.66.39.255 2 128.66.64.0 /23 29 128.66.65.255 3 192.51.100.0 /22 210 192.51.103.255 4 192.51.96.0 /20 212 192.51.111.255 5 128.66.1.128 /26 26 128.66.1.191 6 128.66.3.96 /28 24 128.66.3.111 7 192.51.100.192 /29 23 192.51.100.199 8 192.51.100.64 /27 25 192.51.100.95 18 CCENT/CCNA ICND1 100-105 Official Cert Guide

Table N-7 lists the answers to questions posed earlier in Table N-2.

Table N-7 CIDR Block Practice: Size of Block and Range of Numbers CIDR Block ID CIDR Block Subnet S (Subnet H (Host # Subnets # Hosts Length Mask Bits) Bits) 1 128.66.32.0 /21 /24 3 8 8 252 2 128.66.64.0 /23 /26 3 6 8 62 3 192.51.100.0 /22 /28 6 4 64 14 4 192.51.96.0 /20 /25 5 7 32 126 5 128.66.1.128 /18 /23 5 9 32 510 6 128.66.3.96 /19 /25 6 7 64 126 7 192.51.100.192 /24 /26 2 6 4 62 8 192.51.100.64 /27 /29 2 3 4 6