 or halve numbers. Subnet masks, like all things computer-related, are based on binary. Because binary is based on powers of two, all you really need to be able to do is to double a number or cut it in half. Memorize the nine possible values for subnet masks (two are easy-0 and 255), and write them down on a sheet of paper (as shown in Figure 36-14). Once you have those down, you've successfully done all the memorization you need. Value Figure 36-14. Subnet worksheet step #1 You'll fill in the rest of the sheet by using the simplest math possible: halving and doubling. Write the numbers 1:256 next to the 0 at the top. This is easy to remember. How many natural networks are there in a Class C? 1. How many hosts are there in a Class C? 256. Remember that everything regarding subnets comes back to the number Now, take the number to the left of the colon, and double it (1 * 2 = 2), and take the number to the right of the colon, and cut it in half (256 / 2 = 128). Repeat this process until you get to the bottom of the sheet. You've just figured out the network:host ratios for a native Class C network! Your paper should look like the example in Figure 36-15. Chapter 36: IP Design 4th Octet Value 1:256 Double this Halve this number number Figure 36-15. Subnet worksheet step #2 This trick is SO simple that when I take mentally exhausting exams like the CCIE lab, actually write out a sheet like this SO I don't have to think at all during the lab. Now that you have all the ratios, you're set for working on a Class C network-but what about other networks? Well, you have two choices: always carry this book with you (which certainly would appeal to me) or apply some simple logic to what you've already learned. As an example, let's use the private network 172.16.0.0 255.255.0.0, which is /16. People often get confused when working with ranges other than /24 because these ranges aren't usually seen in small companies. In practice, when you're using this method, there is no difference. We'll divide the network into eight pieces.