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.