You do not need to memorize the chart, however, if you add one more bit of math to the process we used to calculate the prefix and broadcast address in the preceding section. To understand this method, you need to understand why the skip chart works. Figure 2-8 illustrates.

Figure 2-8 Binary Places in the IPv4 Address

Notice the numbers below each bit; these are the binary places, which are just like the 1s, 10s, 100s, etc., in the decimal number system everyone learns in school. If any of these change to either a 0 or 1, the entire number changes value by the amount shown below:

These numbers are the powers of two from 20 to 27.

Counting over the number of bits in the prefix length—26—we come to the second bit in the fourth octet, which is a 1. If this bit changes to a 0, the value of the number changes by 64, so 64 is the skip value. Networks with a 26-bit prefix length can exist only on boundaries of 64—0, 64, 128, and 192—with a 26-bit prefix length. Because the 26th bit is in the fourth octet, the networks will count by 64s in the fourth octet. The study guide is at https://www.acedexam.com/300-425-enwlsd-designing-cisco-enterprise-wireless-networks/

If you can find the correct octet from the prefix length and then figure out what the skip is, you can calculate the prefix and broadcast address without the chart. Using 198.51.100.70/26 as an example again:

Divide 8 into the prefix length; ignore the remainder and add 1. In this case, 26/8 is 3; we add 1 and find we are working in the fourth octet of the IPv4 address.

Multiply the working octet by 8; subtract the prefix length. In this case, 8*4 is 32, and subtracting 26 from 32 gives us 6.

Find the power of 2 of this number; in this case, 2^6 is 64. Find the prefix. In this case, 64 will go into 70 once, and we’re working in the fourth octet, so the prefix is 198.51.100.64.

Subtract 1 from the skip and add it to the prefix to find the broadcast address. In this case, the skip is 64. Subtracting 1, we get 63, and adding to 64, we get 127, so the broadcast address is 198.51.100.127.

Again, this method takes some practice to remember all the steps, but it reduces the entire problem to some simple division (without remainders), multiplication, addition, and subtraction. With some practice, you can use this technique to quickly find prefixes and broadcast addresses.

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