et in a subnet mask. They are: 0, 128, 192, 224, 240, 248, 252, 254, and 255. IP Subnetting Made Easy Subnet masks are, by their nature, inclusive. That is to say, you can only add or sub- tract 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 34-10 shows bits validly and invalidly used, with their resulting decimal octets. Valid subnet octets Invalid subnet octets 10000000 01000000 11000000 10100000 11100000 11010000 Figure 34-10. 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 unobvious ways, you should never do it outside of 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 34-11. Binary Ratio 0000 0000 1:256 1000 0000 2:128 1100 0000 1110 0000 1111 0000 16:16 1111 1000 1111 1100 1111 1110 128:2 1111 1111 256:1 Figure 34-11. Possible subnet octet values Chapter 34: IP Design 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 34-12, I've laid out the ratios as they apply to any octet in any position in the subnet mask. None of this needs to be memorized, 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 posi- tion in the subnet mask, you can figure out what the octet should be. First octet Second octet Third octet Fourth octet 256*256*256*256 2561 1 256*256*256 256*256*1 256*256 256*256*256*1 128*256*256*256 256* 2 128*256*256 256*256*2 128*256 256*256*256*2 64*256*256*256 256* 4 64*256*256 256*256*4 64*256 256*256*2