 for an octet is 255. Remember, though, that the values start with 0, SO 255 is really the 256th number possible. This is a very important concept, because everything else is predicated on it. IP Subnetting Made Easy The second rule astounds many people. In fact, it astounded me when I discovered it while writing my own subnet calculator program: Only nine values are possible for any octet in a subnet mask. They are: 0, 128, 192, 224, 240, 248, 252, 254, and 255. Subnet masks are, by their nature, inclusive. That is to say, you can only add or subtract bits from the mask in bit order, and they must be added from left to right-you cannot skip a bit in a subnet mask. Figure 36-10 shows bits validly and invalidly used, with their resulting decimal octets. Valid subnet octets Invalid subnet octets 10000000 01000000 11000000 10100000 11100000 11010000 Figure 36-10. Valid and invalid subnet octets The CCIE exam requires knowledge of manipulating subnet masks in unnatural ways, but you should never see this in the real world. While it is possible to use subnet masks in nonobvious ways, you should never do it outside a lab environment. Because there are only eight bits in an octet, if you limit yourself to only allowing bits to be added or subtracted, there can only be a finite number of values represented in binary. Those values are shown in Figure 36-11. Notice the column titled Ratio. This is something you won't see in the manufacturers' texts, which is a shame, because I believe it is the very essence of subnet masks in the real world. In Figure 36-12, I've laid out the ratios as they apply to any octet in any position in the subnet mask. You don't need to memorize any of this, as it will all become clear shortly. Notice the patterns at work here. The ratio you will learn to use for a Class C network works for any class network. Simply by moving the ratio to the proper position in the subnet mask, you can figure out what the octet should be. 664 I Chapter 36: IP Design Binary Ratio 0000 0000 1:256 1000 0000 2:128 110