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(c)Find the multiple of the magic number that is closest to but not greater than the address’s interesting octet.

(d)Write down that multiple of the magic number as the value of the subnet number’s interesting octet.

(e)To find the first valid IP address in the subnet, copy the subnet number, but add 1 to the fourth octet.

Step 3 Derive the broadcast address and the last valid IP address:

(a)Write down 255s in the broadcast address octets to the right of the line or the box.

(b)To find the broadcast address’s interesting octet value, take the subnet number’s interesting octet value, add the magic number, and subtract 1.

(c)To find the last valid IP address in the subnet, copy the broadcast address, but subtract 1 from the fourth octet.

Becoming proficient at this shortcut takes some practice. To make sure you have the process down, review the examples in the CD chapter “Subnetting Practice: 25 Subnetting Questions,” which has 25 different examples, including the Boolean AND and shortcut methods of finding the subnet number.

Which Subnet Masks Meet the Stated Design Requirements?

This chapter has explained how to answer questions that provide the subnet number. However, some questions do not supply the subnet number, but instead ask you to choose the correct subnet mask given a set of requirements. The most common of these questions reads something like this:

“You are using Class B network X, and you need 200 subnets, with at most 200 hosts per subnet. Which of the following subnet masks can you use?” This is followed by some subnet masks from which you choose the answer.

To find the correct answers to these types of questions, you first need to decide how many subnet bits and host bits you need to meet the requirements. Basically, the number of hosts per subnet is 2x – 2, where x is the number of host bits in the address. Likewise, the number of subnets in a network, assuming that the same subnet mask is used all over the network, is also 2x – 2, but with x being the number of subnet bits. As soon as you know how many subnet bits and host bits are required, you can figure out what mask or masks meet the stated design goals in the question.

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Examples certainly help. The first sample question reads like this:

“Your network can use Class B network 130.1.0.0. What subnet masks meet the requirement that you plan to allow at most 200 subnets, with at most 200 hosts per subnet?”

First you need to figure out how many subnet bits allow for 200 subnets. You can use the formula 2x – 2 and plug in values for x until one of the numbers is at least 200. In this case, x turns out to be 8. In other words, you need at least 8 subnet bits to allow for 200 subnets.

If you do not want to keep plugging values into the 2x – 2 formula, you can instead memorize Table 4-30.

Table 4-30 Maximum Number of Subnets/Hosts

As you can see, if you already have the powers of 2 memorized, you really do not need to memorize the table—just remember the formula.

As for the first sample question, 7 subnet bits is not enough, because that allows for only 126 subnets. You need 8 subnet bits. Similarly, because you need up to 200 hosts per subnet, you need 8 host bits.

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Finally, you need to decide what mask(s) to use, knowing that you have a Class B network and that you must have at least 8 subnet bits and 8 host bits. Using the letter N to represent network bits, the letter S to represent subnet bits, and the letter H to represent host bits, the following shows the sizes of the various fields:

NNNNNNNN NNNNNNNN SSSSSSSS HHHHHHHH

All that is left is to derive the actual subnet mask. Because you need 8 bits for the subnet field and 8 for the host field, and the network field takes up 16 bits, you have already allocated all 32 bits of the address structure. Therefore, only one possible subnet mask works. To figure out the mask, you need to write down the 32-bit subnet mask, applying the following fact and subnet masks:

The network and subnet bits in a subnet mask are, by definition, all binary 1s.

Similarly, the host bits in a subnet mask are, by definition, all binary 0s.

So, the only valid subnet mask, in binary, is

11111111 11111111 11111111 00000000

When converted to decimal, this is 255.255.255.0.

A second example shows how the requirements stated in the question allow for multiple possible subnet masks:

“Your network can use Class B network 130.1.0.0. What subnet masks meet the requirement that you plan to allow at most 50 subnets, with at most 200 hosts per subnet?”

For this design, you still need at least 8 host bits, but now you only need at least 6 subnet bits. 6 subnet bits allows for 26 – 2, or 62, subnets. Following the same convention as before, but now using an X for bits that can be either subnet or host bits, the format of the address structure is as follows:

NNNNNNNN NNNNNNNN SSSSSSXX HHHHHHHH

In other words, the addresses have 16 network bits, at least 6 subnet bits, and at least 8 host bits. This example actually allows for three valid subnet masks, whose structure is as follows:

NNNNNNNN NNNNNNNN SSSSSSSS HHHHHHHH – 8 subnet, 8 host NNNNNNNN NNNNNNNN SSSSSSSH HHHHHHHH – 7 subnet, 9 host NNNNNNNN NNNNNNNN SSSSSSHH HHHHHHHH – 6 subnet, 10 host

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So, based on the requirements in the question, three different valid subnet masks meet the requirements:

11111111 11111111 11111111 00000000 255.255.255.0

11111111 11111111 11111110 00000000 255.255.254.0

11111111 11111111 11111100 00000000 255.255.252.0

The 2 bits that could be subnet bits or host bits, based on the requirements, are shown in bold.

What Are the Other Subnet Numbers?

The final general type of IP addressing and subnetting question covered in this chapter asks you to list all the subnets of a particular network. You could use a long process that requires you to count in binary and convert many numbers from binary to decimal. However, because most people either learn the shortcut or use a subnet calculator in their jobs, I decided to just show you the shortcut method for this particular type of question.

First, the question needs a better definition—or at least a more-complete one. The question might be better stated like this:

“If the same subnet mask is used for all subnets of this Class A, B, or C network, what are the valid subnets?”

IP design conventions do not require the engineer to use the same mask for every subnet. Unless specifically stated, the question “What are all the subnets?” probably assumes that the same mask is used for all subnets, unless the question specifically states that different masks can be used on different subnets.

The following easy decimal process lists all the valid subnets given the network number and the only mask used on that network. This three-step process assumes that the size of the subnet part of the address is, at most, 8 bits in length. The same general process can be expanded to work when the size of the subnet part of the address is more than 8 bits, but that expanded process is not described here.

The three-step process uses a chart that I call the subnet list chart. I made up the name just for this book. Table 4-31 presents a generic version of the subnet list chart.

Table 4-31 Three-Step Process Generic Subnet List Chart

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Table 4-31 Three-Step Process Generic Subnet List Chart (Continued)

You list the known network number and subnet mask as the first step in the process. If the question gives you an IP address and mask instead of the network number and mask, just write down the network number of which that IP address is a member. (Remember, this three-step process assumes that the subnet part of the addresses is 8 bits or less.)

For the second of the three steps, copy the network number into the Subnet Zero row. Subnet zero, or the zero subnet, is numerically the first subnet, and it is one of the two reserved subnet numbers in a network. (You can use the zero subnet on a Cisco router if you configure the global configuration command ip zero-subnet.) Interestingly, a network's zero subnet has the exact same numeric value as the network itself—which is one of the reasons that it should not be used. For the purposes of answering questions on the exam about the number of valid subnets in a network, consider the zero subnet unusable, unless the question tells you that using it is ok. In real life, do not use the zero subnet if you do not have to.

The third step in the process is covered after Tables 4-32 and 4-33, which list two familiar examples, with the first two steps completed.

Table 4-32 Subnet List Chart: 130.4.0.0/24

Table 4-33 Subnet List Chart: 130.4.0.0/22

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You probably wouldn’t guess that 130.4.252.0 is the broadcast subnet for this latest example. However, adding the magic number 4 to 252 gives you 256 as the next subnet number, which is invalid, so 130.4.252.0 is indeed the broadcast subnet.

The three-step process to find all the subnet numbers of a network is as follows:

Step 1 Write down the network number and subnet mask in the first two rows of the subnet list chart.

Step 2 Write down the network number in the third row. This is the zero subnet, which is one of the two reserved subnets.

Step 3 Do the following two tasks, stopping when the next number you would write down in the interesting column is 256. (But don’t write it down— it’s invalid.)

(a)Copy all three uninteresting octets from the previous line.

(b)Add the magic number to the previous interesting octet, and write it down as the value of the interesting octet.