CompTIA Network+ Study Guide (Sybex Study Guide)-0325-0360.pdf

Chapter 8 IP Subnetting, Troubleshooting IP, and Introduction to NAT THE FOLLOWING COMPTIA NETWORK+ EXAM OBJECTIVES ARE COVERED IN THIS CHAPTER: Domain 1.0 Networking Concepts 1.4 Explain common networking ports, protocols, services, and traffic types. Traffic types Unicast Mul...

Chapter 8 IP Subnetting, Troubleshooting IP, and Introduction to NAT THE FOLLOWING COMPTIA NETWORK+ EXAM OBJECTIVES ARE COVERED IN THIS CHAPTER: Domain 1.0 Networking Concepts 1.4 Explain common networking ports, protocols, services, and traffic types. Traffic types Unicast Multicast Anycast Broadcast 1.7 Given a scenario, use appropriate IPv4 network addressing. Public vs. private Automatic Private IP Addressing (APIPA) RFC1918 Loopback/localhost Subnetting Variable Length Subnet Mask (VLSM) Classless Inter-domain Routing (CIDR) IPv4 address classes Class A Class B Class C Class D Class E Domain 2.0 Network Implementation 2.1 Explain characteristics of routing technologies. Address translation NAT  Port address translation (PAT) This chapter's focus will really zoom in on IP addressing to ensure that you have it nailed down tight. This is an integral aspect of networking, and it's important to your success on the exams and as a professional, too! We'll start with subnetting an IP network. You're going to have to really apply yourself because it takes time and practice to do subnetting correctly and quickly. So, be patient and do whatever it takes to get this stuff dialed in. This chapter truly is important— possibly the most important chapter in this book for you to understand. Make it part of you! I'll thoroughly cover IP subnetting from the very beginning. I know this might sound weird to you, but I think you'll be much better off if you can try to forget everything you've learned about subnetting before reading this chapter—especially if you've been to a Microsoft class! I'll also take you through IP address troubleshooting and walk you through each of the steps recommended when you're faced with troubleshooting an IP network. Finally, I'll finish up with an introduction to network address translation (NAT)—there are various types of NAT, and you need to know when you would use each one. So get psyched—you're about to go for quite a ride! This chapter will truly help you understand IP addressing and networking, so don't get discouraged or give up. If you stick with it, I promise that one day you'll look back on this and be really glad you decided to stay the course. It's one of those things that after you understand it, you'll laugh at that time, way back when, when you thought this was hard. So, are you ready now? Let's go! To find Todd Lammle CompTIA videos and practice questions, please see www.lammle.com. Subnetting Basics In Chapter 7, “IP Addressing,” you learned how to define and find the valid host ranges used in a Class A, Class B, or Class C network address by turning the host bits all off and then all on. This is very good, but here's the catch: You were defining only one network. What would happen if you wanted to take one network address range and create six networks from it? You would have to do something called subnetting, because that's what allows you to take one larger network and break it into a bunch of smaller networks. There are loads of reasons in favor of subnetting, including the following benefits: Reduced Network Traffic We all appreciate less traffic of any kind. With networks, it's no different. Without trusty routers, packet traffic could grind the entire network down to a near standstill. With routers, most traffic will stay on the local network; only packets destined for other networks will pass through the router. Routers create broadcast domains. The more broadcast domains you create, the smaller the broadcast domains and the less network traffic on each network segment. Optimized Network Performance This is the very cool reward you get when you reduce network traffic! Simplified Management It's easier to identify and isolate network problems in a group of smaller connected networks than within one gigantic network. Facilitated Spanning of Large Geographical Distances Because WAN links are considerably slower and more expensive than LAN links, a single large network that spans long distances can create problems in every area previously listed. Connecting multiple smaller networks makes the system more efficient. Next, we're going to move on to subnetting a network address. This is the good part— ready? How to Create Subnets To create subnetworks, you take bits from the host portion of the IP address and reserve them to define the subnet address. This means fewer bits for hosts, so the more subnets, the fewer bits are left available for defining hosts. Soon, I'll show you how to create subnets, starting with Class C addresses. But before you actually implement subnetting, you really need to determine your current requirements as well as plan for future conditions. Follow these steps—they're your recipe for solid design: 1. Determine the number of required network IDs: One for each subnet One for each wide area network (WAN) connection 2. Determine the number of required host IDs per subnet: One for each TCP/IP host One for each router interface 3. Based on the previous requirements, create the following: One subnet mask for your entire network A unique subnet ID for each physical segment A range of host IDs for each subnet Understanding the Powers of 2 By the way, powers of 2 are really important to memorize for use with IP  subnetting. To review powers of 2, remember that when you see a number with another number to its upper right (an exponent), this means you should multiply the number by itself as many times as the upper number specifies. For example, 23 is 2 × 2 × 2, which equals 8. Here's a list of powers of 2 that you should commit to memory: 21 = 2 22 = 4 23 = 8 24 = 16 25 = 32 26 = 64 27 = 128 28 = 256 29 = 512 210 = 1,024 211 = 2,048 212 = 4,096 213 = 8,192 214 = 16,384 If you hate math, don't get stressed out about knowing all these exponents—it's helpful to know them, but it's not absolutely necessary. Here's a little trick, because you're working with 2s: Each successive power of 2 is double the previous one. For example, all you have to do to remember the value of 29 is to first know that 28 = 256. Why? Because when you double 2 to the eighth power (256), you get 29 (or 512). To determine the value of 210, simply start at 28 = 256, and then double it twice. You can go the other way as well. If you needed to know what 26 is, for example, you just cut 256 in half two times: once to reach 27 and then one more time to reach 26. Not bad, right? Subnet Masks For the subnet address scheme to work, every machine on the network must know which part of the host address will be used as the subnet address. This is accomplished by assigning a subnet mask to each machine. A subnet mask is a 32-bit value that allows the recipient of IP packets to distinguish the network ID portion of the IP address from the host ID portion of the IP address. The network administrator creates a 32-bit subnet mask composed of 1s and 0s. The 1s in the subnet mask represent the positions that refer to the network, or subnet, addresses. Not all networks need subnets, meaning they use the default subnet mask. This is basically the same as saying that a network doesn't have a subnet address. Table 8.1 shows the default subnet masks for Classes A, B, and C. These default masks cannot and do not change. In other words, you can't make a Class B subnet mask read 255.0.0.0. If you try, the host will read that address as invalid and usually won't even let you type it in. For a Class A network, you can't change the first byte in a subnet mask; it must read 255.0.0.0 at a minimum. Similarly, you cannot assign 255.255.255.255, because this is all 1s—a broadcast address. A Class B address must start with 255.255.0.0, and a Class C has to start with 255.255.255.0. Check out Table 8.1. TABLE 8.1 Default subnet masks Default Subnet ClassFormat Mask A network.host.host.host 255.0.0.0 B network.network.host.host 255.255.0.0 C network.network.network.host255.255.255.0 In Chapter 7 we discussed the addresses with the first octet of 224 to 255 are reserved for Class D and E networks. Class D (224–239) is used for multicast addresses and Class E (240–255) for scientific purposes. But they're really beyond the scope of this book, so I'm not going to go into detail about them here. But you do need to remember that the multicast range is from 224.0.0.0 through 239.255.255.255. Classless Inter-Domain Routing (CIDR) Another term you need to know is Classless Inter-Domain Routing (CIDR). It's basically the method that Internet service providers (ISPs) use to allocate a number of addresses to a company or a home connection. They provide addresses in a certain block size; I'll be going into that in greater detail later in this chapter. Another term for the use of different length subnet masks in the network is variable-length subnet masking (VLSM). When you receive a block of addresses from an ISP, what you get will look something like this: 192.168.10.32/28. This is telling you what your subnet mask is. The slash notation (/) means how many bits are turned on (1s). Obviously, the maximum could only be /32 because a byte is 8 bits and there are 4 bytes in an IP address: 4 × 8 = 32. But keep in mind that the largest subnet mask available (regardless of the class of address) can only be a /30 because you have to keep at least 2 bits for host bits. Take, for example, a Class A default subnet mask, which is 255.0.0.0. This means that the first byte of the subnet mask is all ones (1s), or 11111111. When referring to a slash notation, you need to count all the 1 bits to figure out your mask. The 255.0.0.0 is considered a /8 because it has 8 bits that are 1s—that is, 8 bits that are turned on. A Class B default mask would be 255.255.0.0, which is a /16 because 16 bits are (1s): 11111111.11111111.00000000.00000000. Table 8.2 lists every available subnet mask and its equivalent CIDR slash notation. TABLE 8.2 CIDR values Subnet Mask CIDR Value 255.0.0.0 /8 255.128.0.0 /9 255.192.0.0 /10 255.224.0.0 /11 255.240.0.0 /12 255.248.0.0 /13 255.252.0.0 /14 255.254.0.0 /15 255.255.0.0 /16 255.255.128.0 /17 255.255.192.0 /18 255.255.224.0 /19 255.255.240.0 /20 255.255.248.0 /21 255.255.252.0 /22 255.255.254.0 /23 255.255.255.0 /24 255.255.255.128 /25 255.255.255.192 /26 255.255.255.224 /27 255.255.255.240/28 255.255.255.248/29 255.255.255.252 /30 Although, according to RFC 1518, any device or software that claims to be CIDR- compliant will allow supernetting, meaning a traditional Class C address can be used with a /23 subnet mask, in almost all cases. The /8 through /15 can be used only with Class A network addresses; /16 through /23 can be used by Class A and B network addresses; and /24 through /30 can be used by Class A, B, and C network addresses. This is a big reason most companies use Class A network addresses. By being allowed the use of all subnet masks, they gain the valuable benefit of maximum flexibility for their network design.  Supernetting is the opposite of subnetting. Subnetting is the division of a network into subnets. Supernetting is the method of combining smaller networks in a single address. EXERCISE 8.1 Examining IP Address and Subnet Masks In this exercise, you will examine your IP address and subnet mask. 1. Open the command prompt by clicking Start, typing cmd, and then pressing Enter. 2. In the command prompt, type ipconfig /all and make note of the IP address and subnet mask. 3. Examine how many bits your subnet mask is using and write it down in CIDR notation. 4. Identify the network ID portion of your IP address and write it down. 5. Identify the host ID portion of the IP address and write it down. 6. Identify the class of IP address being used on your computer by examining the first octet. After examining your IP address and subnet mask, you should have a good understanding of the subnet mask compared to the class of IP address. You should also know the network that the host belongs to and CIDR notation. Subnetting Class C Addresses There are many different ways to subnet a network. The right way is the way that works best for you. In a Class C address, only 8 bits are available for defining the hosts. Remember that subnet bits start at the left and go to the right, without skipping bits. This means the only Class C subnet masks can be those listed here: Binary DecimalCIDR 000000000 /24 10000000 128 /25 11000000 192 /26 11100000 224 /27 11110000 240 /28 11111000 248 /29  11111100 252 /30 We can't use a /31 or /32 because, remember, we have to leave at least 2 host bits for assigning IP addresses to hosts. Get ready for something special. I'm going to teach you an alternate method of subnetting that makes it a whole lot easier to subnet larger numbers in no time. Trust me, you really do need to be able to subnet fast! Subnetting a Class C Address: The Fast Way! When you've chosen a possible subnet mask for your network and need to determine the number of subnets, valid hosts, and broadcast addresses of a subnet that the mask provides, all you need to do is answer five simple questions: How many subnets does the chosen subnet mask produce? How many valid hosts per subnet are available? What are the valid subnets? What's the broadcast address of each subnet? What are the valid hosts in each subnet? At this point, it's important that you both understand and have memorized your powers of 2. Please refer to the sidebar “Understanding the Powers of 2” earlier in this chapter if you need some help. Here's how you get the answers to those five big questions: How many subnets? 2x = number of subnets. x is the number of masked bits, or the 1s. For example, in 11000000, the number of 1s gives us 22 subnets. In this example, there are four subnets. How many hosts per subnet? 2y – 2 = number of hosts per subnet. y is the number of unmasked bits, or the 0s. For example, in 11000000, the number of 0s gives us 26 – 2 hosts. In this example, there are 62 hosts per subnet. You need to subtract 2 for the subnet address and the broadcast address, which are not valid hosts. What are the valid subnets? 256 – subnet mask = block size, or increment number. An example would be 256 – 192 = 64. The block size of a 192 mask is always 64. Start counting at zero in blocks of 64 until you reach the subnet mask value, and these are your subnets. 0, 64, 128, 192. Easy, huh? What's the broadcast address for each subnet? Now here's the really easy part. Because we counted our subnets in the last section as 0, 64, 128, and 192, the broadcast address is always the number right before the next subnet. For example, the 0 subnet has a broadcast address of 63 because the next subnet is 64. The 64 subnet has a broadcast address of 127 because the next subnet is 128. And so on. And remember, the broadcast address of the last subnet is always 255. What are the valid hosts? Valid hosts are the numbers between the subnets, omitting all the 0s and all the 1s. For example, if 64 is the subnet number and 127 is the broadcast address, then 65–126 is the valid host range—it's always the numbers  between the subnet address and the broadcast address. I know this can truly seem confusing. But it really isn't as hard as it seems to be at first— just hang in there! Why not try a few and see for yourself? Subnetting Practice Examples: Class C Addresses Here's your opportunity to practice subnetting Class C addresses using the method I just described. Exciting, isn't it? We're going to start with the first Class C subnet mask and work through every subnet that we can using a Class C address. When we're done, I'll show you how easy this is with Class B networks too! Practice Example #1C: 255.255.255.128 (/25) Because 128 is 10000000 in binary, there is only 1 bit for subnetting, and there are 7 bits for hosts. We're going to subnet the Class C network address 192.168.10.0. 192.168.10.0 = Network address 255.255.255.128 = Subnet mask Now, let's answer the big five: How many subnets? Because 128 is 1 bit on (10000000), the answer is 21 = 2. How many hosts per subnet? We have 7 host bits off (10000000), so the equation is 27 – 2 = 126 hosts. What are the valid subnets? 256 – 128 = 128. Remember, we'll start at zero and count in our block size, so our subnets are 0, 128. What's the broadcast address for each subnet? The number right before the value of the next subnet is all host bits turned on and equals the broadcast address. For the 0 subnet, the next subnet is 128, so the broadcast address of the 0 subnet is 127. What are the valid hosts? These are the numbers between the subnet and broadcast address. The easiest way to find the hosts is to write out the subnet address and the broadcast address. This way, the valid hosts are obvious. The following table shows the 0 and 128 subnets, the valid host ranges of each, and the broadcast address of both subnets: Subnet 0 128 First host 1 129 Last host 126254 Broadcast127 255 Before moving on to the next example, take a look at Figure 8.1. Okay, looking at a Class C /25, it's pretty clear there are two subnets. But so what—why is this significant? Well actually, it's not, but that's not the right question. What you really want to know is what you would do with this information! The key to understanding subnetting is to understand the very reason you need to do it. And I'm going to demonstrate this by going through the process of building a physical network—and let's add a router. (We now have an internetwork, as I truly hope you already know!) Because we added that router, for the hosts on our internetwork to communicate, they must now have a logical network addressing scheme. We could use IPv6, but IPv4 is still the most popular, and it also just happens to be what we're studying at the moment, so that's what we're going with. Now take a look back at Figure 8.1. By the way, the output you see after the diagram is the routing table of the router, which was displayed by executing the show ip route command on the router. There are two physical networks, so we're going to implement a logical addressing scheme that allows for two logical networks. As always, it's a really good idea to look ahead and consider any likely growth scenarios—both short and long term, but for this example, a /25 will do the trick. FIGURE 8.1 Implementing a Class C /25 logical network Practice Example #2C: 255.255.255.192 (/26) In this second example, we're going to subnet the network address 192.168.10.0 using the subnet mask 255.255.255.192. 192.168.10.0 = Network address 255.255.255.192 = Subnet mask It's time to answer the big five: How many subnets? Because 192 is 2 bits on (11000000), the answer is 22 = 4 subnets. How many hosts per subnet? We have 6 host bits off (11000000), so the equation is 26 – 2 = 62 hosts. What are the valid subnets? 256 – 192 = 64. Remember, we start at zero and count in our block size, so our subnets are 0, 64, 128, and 192. What's the broadcast address for each subnet? The number right before the value of the next subnet is all host bits turned on and equals the broadcast address. For the 0 subnet, the next subnet is 64, so the broadcast address for the 0 subnet is 63. What are the valid hosts? These are the numbers between the subnet and broadcast address. The easiest way to find the hosts is to write out the subnet address and the  broadcast address. This way, the valid hosts are obvious. The following table shows the 0, 64, 128, and 192 subnets, the valid host ranges of each, and the broadcast address of each subnet: The subnets (do this first) 0 64 128192 Our first host (perform host addressing 1 65 129 193 last) Our last host 62126190 254 The broadcast address (do this second) 63127 191 255 Again, before getting into the next example, you can see that we can now subnet a /26. And what are you going to do with this fascinating information? Implement it! We'll use Figure 8.2 to practice a /26 network implementation. FIGURE 8.2 Implementing a Class C /26 logical network The /26 mask provides four subnetworks, and we need a subnet for each router interface. With this mask, in this example, we actually have room to add another router interface. Practice Example #3C: 255.255.255.224 (/27) This time, we'll subnet the network address 192.168.10.0 and subnet mask 255.255.255.224. 192.168.10.0 = Network address 255.255.255.224 = Subnet mask How many subnets? 224 is 11100000, so our equation is 23 = 8.  How many hosts? 25 – 2 = 30. What are the valid subnets? 256 – 224 = 32. We just start at zero and count to the subnet mask value in blocks (increments) of 32: 0, 32, 64, 96, 128, 160, 192, and 224. What's the broadcast address for each subnet (always the number right before the next subnet)? What are the valid hosts (the numbers between the subnet number and the broadcast address)? To answer the last two questions, first just write out the subnets, and then write out the broadcast address—the number right before the next subnet. Last, fill in the host address. The following table gives you all the subnets for the 255.255.255.224 Class C subnet mask: The subnet 0 326496 128160192224 address The first valid host 1 33 65 97 129 161 193 225 The last valid host 3062 94 126158 190 222 254 The broadcast address 31 63 95 127 159 191 223 255 Practice Example #4C: 255.255.255.240 (/28) Let's practice on another one: 192.168.10.0 = Network address 255.255.255.240 = Subnet mask Subnets? 240 is 11110000 in binary. 24 = 16. Hosts? 4 host bits, or 24 – 2 = 14. Valid subnets? 256 – 240 = 16. 0, 16, 32, 48, 64, 80, 96, 112, 128, 144, 160, 176, 192, 208, 224, 240. Broadcast address for each subnet? Valid hosts? To answer the last two questions, check out the following table. It gives you the subnets, valid hosts, and broadcast address for each subnet. First, find the address of each subnet using the block size (increment). Second, find the broadcast address of each subnet increment (it's always the number right before the next valid subnet); then, just fill in the host address. The following table shows the available subnets, hosts, and broadcast address provided by a Class C network 255.255.255.240 mask: Subnet 0 16 3248648096 112128144160176192208224240 First host 1 17 33 49 65 81 97 113 129 145 161 177 193 209 225 241 Last host 143046 62 78 94 110126 142 158 174 190 206 222 238 254  Broadcast1531 47 63 79 95 111 127 143 159 175 191 207 223 239 255 Practice Example #5C: 255.255.255.248 (/29) Let's keep practicing: 192.168.10.0 = Network address 255.255.255.248 = Subnet mask Subnets? 248 in binary = 11111000. 25 = 32. Hosts? 23 – 2 = 6. Valid subnets? 256 – 248 = 8, start at zero: 0, 8, 16, 24, 32, 40, 48, 56, 64, 72, 80, 88, 96, 104, 112, 120, 128, 136, 144, 152, 160, 168, 176, 184, 192, 200, 208, 216, 224, 232, 240, and 248. Broadcast address for each subnet? Valid hosts? Take a look at the following table. It shows some of the subnets (first four and last four only), valid hosts, and broadcast address for the Class C 255.255.255.248 mask: Subnet 08 1624…224232240248 First host 1 9 17 25 … 225 233 241 249 Last host 6 142230 … 230 238 246 254 Broadcast7 152331 … 231 239 247 255 Practice Example #6C: 255.255.255.252 (/30) I know, I know—but just one more: 192.168.10.0 = Network address 255.255.255.252 = Subnet mask Subnets? 64. Hosts? 2. Valid subnets? 0, 4, 8, 12, and so on, all the way to 252. Broadcast address for each subnet (always the number right before the next subnet)? Valid hosts (the numbers between the subnet number and the broadcast address)? The following table shows you the subnet, valid host, and broadcast address of the first four and last four subnets in the 255.255.255.252 Class C subnet: Subnet 048 12…240244248252 First host 1 5 9 13 … 241 245 249 253 Last host 2 6 1014 … 242 246 250 254 Broadcast3 7 11 15 … 243 247 251 255 Subnetting in Your Head: Class C Addresses It really is possible to subnet in your head by looking at a valid IP address and subnet mask. This is extremely important for IP addressing and troubleshooting. Even if you don't believe me that you can subnet in your head, I'll show you how. And it's not all that hard either—take the following example: What is the subnet, broadcast address, and valid host range that this host IP address is a part of? 192.168.10.33 = Host address 255.255.255.224 = Subnet mask Real World Scenario Should We Really Use This Mask That Provides Only Two Hosts? Imagine you are the network administrator for Acme Corporation in San Francisco, with dozens of WAN links connecting to your corporate office. Right now your network is a classful network, which means that the same subnet mask is on each host and router interface. You've read about classless routing where you can have different size masks, but you don't know what to use on your point-to-point WAN links. Is 255.255.255.252 (/30) a helpful mask in this situation? Yes, this is a very helpful mask in wide area networks. If you use the 255.255.255.0 mask, then each network will have 254 hosts, but you use only two addresses with a WAN link! That is a waste of 252 hosts per subnet. If you use the 255.255.255.252 mask, then each subnet has only two hosts, and you don't waste precious addresses. First, determine the subnet and broadcast address of this IP address. You can do this by answering question 3 of the big five questions: 256 – 224 = 32. Start at zero: 0, 32, 64. The address of 33 falls between the two subnets of 32 and 64 and must be part of the 192.168.10.32 subnet. The next subnet is 64, so the broadcast address of the 32 subnet is 63. (Remember that the broadcast address of a subnet is always the number right before the next subnet.) The valid host range is 33–62 (the numbers between the subnet and broadcast address). I told you this is easy! Okay, let's try another one. What is the subnet, broadcast address, and valid host range that this host IP address is part of? 192.168.10.33 = Host address 255.255.255.240 = Subnet mask 256 – 240 = 16. Start at zero and count till you pass the valid host in the problem: 0, 16, 32, 48. Bingo—the host address is between the 32 and 48 subnets. The subnet is 192.168.10.32, and the broadcast address is 47 (the next subnet is 48). The valid host range is 33–46 (the numbers between the subnet number and the broadcast address). We need to do more, just to make sure you have this down. You have a host address of 192.168.10.174 with a mask of 255.255.255.240. What is the subnet, broadcast address, and valid host range that this host IP address is part of? The mask is 240, so we need our block size: 256 – 240 = 16. Just keep adding 16 until we pass the host address of 174, starting at zero, of course: 0, 16, 32, 48, 64, 80, 96, 112, 128, 144, 160, 176. The host address of 174 is between 160 and 176, so the subnet is 160. The broadcast address is 175; the valid host range is 161–174. That was a tough one. Let's do one more just for fun. This is the easiest one of all Class C subnetting: 192.168.10.17 = Host address 255.255.255.252 = Subnet mask What subnet and broadcast address is this IP address a part of? 256 – 252 = 4. Start at zero (always start at zero unless told otherwise): 0, 4, 8, 12, 16, 20, and so on. You've got it! The host address is between the 16 and 20 subnets. The subnet is 192.168.10.16, and the broadcast address is 19. The valid host range is 17–18. Now that you're all over Class C subnetting, let's move on to Class B subnetting. But before we do, let's do a quick review. So What Do You Know Now? Here's where you can really apply what you've learned so far and begin committing it all to memory. This is a very cool section that I've been using in my classes for years. It will really help you nail down subnetting! When you see a subnet mask or slash notation (CIDR), you should know the following when working with Class C networks. /25 What do you know about a /25? 128 mask 1 bit on and 7 bits off (10000000) Block size of 128 2 subnets, each with 126 hosts /26 And what do you know about a /26? 192 mask 2 bits on and 6 bits off (11000000) Block size of 64  4 subnets, each with 62 hosts /27 What about a /27? 224 mask 3 bits on and 5 bits off (11100000) Block size of 32 8 subnets, each with 30 hosts /28 And what about a /28? 240 mask 4 bits on and 4 bits off Block size of 16 16 subnets, each with 14 hosts /29 What do you know about a /29? 248 mask 5 bits on and 3 bits off Block size of 8 32 subnets, each with 6 hosts /30 And last, what about a /30? 252 mask 6 bits on and 2 bits off Block size of 4 64 subnets, each with 2 hosts Regardless of whether you have a Class A, Class B, or Class C address, the /30 mask will provide you with only two hosts, ever. This mask is suited almost exclusively for use on point-to-point links. If you can memorize this “So What Do You Know Now?” section, you'll be much better off in your day-to-day job and in your studies. Try saying it out loud, which helps you memorize things—yes, your significant other and/or co-workers will think you've lost it, but they probably already do if you're in the networking field. And if you're not yet in the networking field but are studying all this to break into it, you might as well have people start thinking you're a little “different” now because they will eventually anyway. It's also helpful to write these on some type of flashcards and have people test your skill. You'd be amazed at how fast you can get subnetting down if you memorize block sizes as well as this “So What Do You Know Now?” section. Subnetting Class B Addresses Before we dive into this, let's look at all the possible Class B subnet masks. Notice that we have a lot more possible subnet masks than we do with a Class C network address: 255.255.0.0 (/16) 255.255.128.0 (/17) 255.255.192.0 (/18) 255.255.224.0 (/19) 255.255.240.0 (/20) 255.255.248.0 (/21) 255.255.252.0 (/22) 255.255.254.0 (/23) 255.255.255.0 (/24) 255.255.255.128 (/25) 255.255.255.192 (/26) 255.255.255.224(/27) 255.255.255.240(/28) 255.255.255.248(/29) 255.255.255.252 (/30) We know the Class B network address has 16 bits available for host addressing. This means we can use up to 14 bits for subnetting (because we have to leave at least 2 bits for host addressing). Using a /16 means you are not subnetting with Class B, but it is a mask you can use. By the way, do you notice anything interesting about that list of subnet values—a pattern, maybe? Ah-ha! That's exactly why I had you memorize the binary-to-decimal numbers earlier in this chapter. Because subnet mask bits start on the left and move to the right and bits can't be skipped, the numbers are always the same regardless of the class of address. Memorize this pattern. The process of subnetting a Class B network is pretty much the same as it is for a Class C, except that you have more host bits and you start in the third octet. Use the same subnet numbers for the third octet with Class B that you used for the fourth octet with Class C, but add a 0 to the network portion and a 255 to the broadcast section in the fourth octet. The following table shows you an example host range of two subnets used in a Class B 240 (/20) subnet mask: First SubnetSecond Subnet 16.0 32.0 31.255 47.255 Notice that these are the same numbers we used in the fourth octet with a /28 mask, but we moved them to the third octet and added a.0 and.255 at the end. Just add the valid hosts between the numbers, and you're set! Subnetting Practice Examples: Class B Addresses The following sections will give you an opportunity to practice subnetting Class B addresses. Again, I have to mention that this is the same as subnetting with Class C, except we start in the third octet—with the exact same numbers! Practice Example #1B: 255.255.128.0 (/17) Let's take a look at our first example: 172.16.0.0 = Network address 255.255.128.0 = Subnet mask Subnets? 21 = 2 (same as Class C). Hosts? 215 – 2 = 32,766 (7 bits in the third octet, and 8 in the fourth). Valid subnets? 256 – 128 = 128. 0, 128. Remember that subnetting in Class B starts in the third octet, so the subnet numbers are really 0.0 and 128.0, as shown in the next table. These are the exact numbers we used with Class C; we use them in the third octet and add a 0 in the fourth octet for the network address. Broadcast address for each subnet? The following table shows the two subnets available, the valid host range, and the broadcast address of each: Subnet 0.0 128.0 First host 0.1 128.1 Last host 127.254255.254 Broadcast127.255 255.255 Notice that we just added the fourth octet's lowest and highest values and came up with the answers. And again, it's done the same way as for a Class C subnet. We just use the same numbers in the third octet and added 0 and 255 in the fourth octet—pretty simple, huh? I really can't say this enough: It's not hard. The numbers never change. We just use them in different octets! Practice Example #2B: 255.255.192.0 (/18) Let's take a look at a second example with Class B. 172.16.0.0 = Network address  255.255.192.0 = Subnet mask Subnets? 22 = 4. Hosts? 214 – 2 = 16,382 (6 bits in the third octet, and 8 in the fourth). Valid subnets? 256 – 192 = 64. 0, 64, 128, 192. Remember that we're in the third octet, so the subnet numbers are really 0.0, 64.0, 128.0, and 192.0, as shown in the next table. Broadcast address for each subnet? Valid hosts? The following table shows the four subnets available, the valid host range, and the broadcast address of each: Subnet 0.0 64.0 128.0 192.0 First host 0.1 64.1 128.1 192.1 Last host 63.254127.254191.254255.254 Broadcast63.255127.255 191.255255.255 Again, it's pretty much the same as it is for a Class C subnet—we just added 0 and 255 in the fourth octet for each subnet in the third octet. Practice Example #3B: 255.255.240.0 (/20) Let's take a look: 172.16.0.0 = Network address 255.255.240.0 = Subnet mask Subnets? 24 = 16. Hosts? 212 – 2 = 4,094. Valid subnets? 256 – 240 = 16, but we start counting from 0. 0, 16, 32, 48, and so on, up to 240. Notice that these are the same numbers as a Class C 240 mask—we just put them in the third octet and add a 0 and 255 in the fourth octet. Broadcast address for each subnet? Valid hosts? The following table shows the first four subnets, valid hosts, and broadcast address in a Class B 255.255.240.0 mask: Subnet 0.0 16.0 32.0 48.0 First host 0.1 16.1 32.1 48.1 Last host 15.25431.25447.25463.254 Broadcast15.255 31.255 47.25563.255 Practice Example #4B: 255.255.254.0 (/23) Let's take a look: 172.16.0.0 = Network address 255.255.254.0 = Subnet mask Subnets? 27 = 128. Hosts? 29 – 2 = 510. Valid subnets? 256 – 254 = 0, 2, 4, 6, 8, and so on, up to 254. Broadcast address for each subnet? Valid hosts? The following table shows the first five subnets, valid hosts, and broadcast address in a Class B 255.255.254.0 mask: Subnet 0.0 2.0 4.0 6.0 8.0 First host 0.1 2.1 4.1 6.1 8.1 Last host 1.2543.2545.2547.2549.254 Broadcast1.255 3.255 5.2557.255 9.255 Practice Example #5B: 255.255.255.0 (/24) Contrary to popular belief, 255.255.255.0 used with a Class B network address is not called a Class B network with a Class C subnet mask. It's amazing how many people see this mask used in a Class B network and think it's a Class C subnet mask. This is a Class B subnet mask with 8 bits of subnetting—it's considerably different from a Class C mask. Subnetting this address is fairly simple: 172.16.0.0 = Network address 255.255.255.0 = Subnet mask Subnets? 28 = 256. Hosts? 28 – 2 = 254. Valid subnets? 256 – 255 = 1. 0, 1, 2, 3, and so on, all the way to 255. Broadcast address for each subnet? Valid hosts? The following table shows the first four and last two subnets, the valid hosts, and the broadcast address in a Class B 255.255.255.0 mask: Subnet 0.0 1.0 2.0 3.0 …254.0 255.0 First host 0.1 1.1 2.1 3.1 … 254.1 255.1 Last host 0.2541.2542.2543.254… 254.254255.254 Broadcast0.255 1.255 2.255 3.255 … 254.255 255.255 Practice Example #6B: 255.255.255.128 (/25) This is one of the hardest subnet masks you can play with. And worse, it actually is a really good subnet to use in production because it creates more than 500 subnets with a whopping 126 hosts for each subnet—a nice mixture. So, don't skip over it! 172.16.0.0 = Network address 255.255.255.128 = Subnet mask Subnets? 29 = 512. Hosts? 27 – 2 = 126. Valid subnets? Now for the tricky part. 256 – 255 = 1. 0, 1, 2, 3, and so on for the third octet. But you can't forget the one subnet bit used in the fourth octet. Remember when I showed you how to figure one subnet bit with a Class C mask? You figure this out the same way. (Now you know why I showed you the 1-bit subnet mask in the Class C section—to make this part easier.) You actually get two subnets for each third octet value, hence the 512 subnets. For example, if the third octet is showing subnet 3, the two subnets would actually be 3.0 and 3.128. Broadcast address for each subnet? Valid hosts? The following table shows how you can create subnets, valid hosts, and broadcast addresses using the Class B 255.255.255.128 subnet mask (the first eight subnets are shown and then the last two subnets): Subnet 0.0 0.1281.0 1.1282.0 2.1283.0 3.128…255.0 255.128 First host 0.1 0.129 1.1 1.129 2.1 2.129 3.1 3.129 … 255.1 255.129 Last host 0.1260.254 1.1261.254 2.1262.254 3.1263.254 … 255.126255.254 Broadcast0.127 0.255 1.127 1.255 2.127 2.255 3.127 3.255 … 255.127 255.255 Practice Example #7B: 255.255.255.192 (/26) Now, this is where Class B subnetting gets easy. Because the third octet has a 255 in the mask section, whatever number is listed in the third octet is a subnet number. However, now that we have a subnet number in the fourth octet, we can subnet this octet just as we did with Class C subnetting. Let's try it: 172.16.0.0 = Network address 255.255.255.192 = Subnet mask Subnets? 210 = 1024. Hosts? 26 – 2 = 62. Valid subnets? 256 – 192 = 64. The subnets are shown in the following table. Do these numbers look familiar? Broadcast address for each subnet? Valid hosts? The following table shows the first eight subnet ranges, valid hosts, and broadcast address: Subnet 0.0 0.64 0.1280.1921.0 1.64 1.1281.192 First host 0.1 0.65 0.129 0.193 1.1 1.65 1.129 1.193 Last host 0.620.1260.190 0.254 1.621.1261.190 1.254 Broadcast0.630.127 0.191 0.255 1.631.127 1.191 1.255 Notice that for each subnet value in the third octet, you get subnets 0, 64, 128, and 192 in the fourth octet. Practice Example #8B: 255.255.255.224 (/27) This is done the same way as the preceding subnet mask, except that we have more subnets and fewer hosts per subnet available. 172.16.0.0 = Network address 255.255.255.224 = Subnet mask Subnets? 211 = 2048. Hosts? 25 – 2 = 30. Valid subnets? 256 – 224 = 32. 0, 32, 64, 96, 128, 160, 192, 224. Broadcast address for each subnet? Valid hosts? The following table shows the first eight subnets: Subnet 0.0 0.320.640.96 0.1280.1600.1920.224 First host 0.1 0.33 0.65 0.97 0.129 0.161 0.193 0.225 Last host 0.300.62 0.94 0.1260.158 0.190 0.222 0.254 Broadcast0.31 0.63 0.95 0.127 0.159 0.191 0.223 0.255 This next table shows the last eight subnets: Subnet 255.0 255.32255.64255.96255.128255.160255.192255.224 First host 255.1 255.33 255.65 255.97 255.129 255.161 255.193 255.225 Last host 255.30255.62 255.94 255.126255.158 255.190 255.222 255.254 Broadcast255.31 255.63 255.95 255.127 255.159 255.191 255.223 255.255 Subnetting in Your Head: Class B Addresses Are you nuts? Subnet Class B addresses in our heads? It's actually easier than writing it out—I'm not kidding! Let me show you the steps: 1. What subnet and broadcast address is the IP address 172.16.10.33 255.255.255.224 (/27) a member of? The interesting octet is the fourth octet. 256 – 224 = 32. 32 + 32 = 64. Bingo: 33 is  between 32 and 64. However, remember that the third octet is considered part of the subnet, so the answer is the 10.32 subnet. The broadcast is 10.63 because 10.64 is the next subnet. That was a pretty easy one. 2. What subnet and broadcast address is the IP address 172.16.66.10 255.255.192.0 (/18) a member of? The interesting octet is the third octet instead of the fourth octet. 256 – 192 = 64. 0, 64, 128. The subnet is 172.16.64.0. The broadcast must be 172.16.127.255 because 128.0 is the next subnet. Notice in the last example I started counting at zero. This is called ip subnet-zero. It is a command that if executed on a router, allows us to use the zero subnet as our first subnet. This may or may not be enabled on your router. If it is not enabled, then you cannot start counting subnets at zero. Most routers, if not all routers these days, support ip subnet-zero. 3. What subnet and broadcast address is the IP address 172.16.50.10 255.255.224.0 (/19) a member of? 256 – 224 = 0, 32, 64 (remember, we always start counting at zero). The subnet is 172.16.32.0, and the broadcast must be 172.16.63.255 because 64.0 is the next subnet. 4. What subnet and broadcast address is the IP address 172.16.46.255 255.255.240.0 (/20) a member of? 256 – 240 = 16. The third octet is interesting to us. 0, 16, 32, 48. This subnet address must be in the 172.16.32.0 subnet, and the broadcast must be 172.16.47.255 because 48.0 is the next subnet. So, yes, 172.16.46.255 is a valid host. 5. What subnet and broadcast address is the IP address 172.16.45.14 255.255.255.252 (/30) a member of? Where is the interesting octet? 256 – 252 = 0, 4, 8, 12, 16 (in the fourth octet). The subnet is 172.16.45.12, with a broadcast of 172.16.45.15 because the next subnet is 172.16.45.16. 6. What is the subnet and broadcast address of the host 172.16.88.255/20? What is a /20? If you can't answer this, you can't answer this question, can you? A /20 is 255.255.240.0, which gives us a block size of 16 in the third octet, and because no subnet bits are on in the fourth octet, the answer is always 0 and 255 in the fourth octet. 0, 16, 32, 48, 64, 80, 96. Bingo: 88 is between 80 and 96, so the subnet is 80.0 and the broadcast address is 95.255. 7. A router receives a packet on an interface with a destination address of 172.16.46.191/26. What will the router do with this packet?  Discard it. Do you know why? 172.16.46.191/26 is a 255.255.255.192 mask, which gives us a block size of 64. Our subnets are then 0, 64, 128, 192. 191 is the broadcast address of the 128 subnet, so a router, by default, will discard any broadcast packets. Troubleshooting IP Addressing Troubleshooting IP addressing is obviously an important skill because running into trouble somewhere along the way is pretty much a sure thing, and it's going to happen to you. No, I'm not a pessimist; I'm just keeping it real. Because of this nasty fact, it will be great when you can save the day because you can both figure out (diagnose) the problem and fix it on an IP network whether you're at work or at home! Let's use Figure 8.3 as an example of your basic IP trouble—poor Sally can't log into the Windows server. Do you deal with this by calling the Microsoft team to tell them their server is a pile of junk and causing all your problems? Tempting, but probably not such a great idea—let's first double-check our network instead. Check out Figure 8.3. FIGURE 8.3 Basic IP troubleshooting Let's get started by going over the basic troubleshooting steps. They're pretty simple but important nonetheless. Pretend you're at Sally's host and she's complaining that she can't communicate to a server that just happens to be on a remote network: 1. Open a command prompt window on Sally's host, and ping 127.0.0.1. C:\>ping 127.0.0.1 Pinging 127.0.0.1 with 32 bytes of data: Reply from 127.0.0.1: bytes=32 time