you require 1,024 hosts to be live devices on the network, you must have a network capable of supporting 2,048 hosts. This is one of the limitations of using a binary system. Similarly, if you need 8 usable hosts on the network, you must provide 16 host addresses, not 8. Figure 36-17 shows the same information as that presented in the preceding worksheet, but in a different format. Some people respond better to seeing this information in a horizontal rather than a vertical format. I like this model, because it is laid out the same way the bits are laid out in a subnet mask. Use whichever format works for you. There are no tests in this book. Remember, a proper subnet mask can have only one of nine values in any single octet. If you start with a value of 1 on the bottom left and a value of 256 on the top right, then double those numbers under each subsequent value, you can quickly figure out what your subnet mask needs to be based on how many hosts or subnets it needs to have. Chapter 36: IP Design 255.255. Subnets Hosts Double numbers in the direction of the arrows Figure 36-17. Horizontal format of the subnet worksheet Once you have memorized the nine subnet octet values (remember, two are easy!) and can double and halve the rest in your head, you'll find that the numbers become obvi- ous. You'll see all of these numbers over and over if you're around computers for any length of time. They're all powers of two, and when you start to recognize the patterns I've outlined for you, calculating any subnet mask will be only a matter of doing some quick, very simple math in your head. IP Subnetting Made Easy CHAPTER 37 You've no doubt heard the news stating that we will run out of IP addresses in the next half hour or SO. Forgive my glibness, but I've been hearing this for 10 years or more. As I was editing this edition for publication, the Number Resource Or- ganization (NRO) reported that as of February 3, 2011, the free pool of IPv4 address space has been depleted. You can read all about it here: http://www.nro.net/