Four Steps to Answering IP Addressing Questions

You must master IP addressing and subnetting to succeed as a network engineer. To pass the CCNA exam, you must at least be able to answer a few questions about subnetting. For most networking jobs, the ability to think about IP addresses and quickly decipher the structure and meaning of the address is a prerequisite for the job.

The exam will test your abilities with questions that go something like this:

• Given a network number and a mask, how many subnets are there, and how many hosts are there per subnet?

• Given an address and a mask, what is the subnet number?

• Given an address and a mask, what is the subnet broadcast address?

• Given an address and a mask, what are the valid IP addresses in the subnet?

This section teaches a process by which you can ignore the four preceding questions and think about IP addressing and subnetting like you normally would for your job. Coincidentally, if you follow the four-step process in this section, you would answer all of the preceding questions without much binary math. This four-step process is designed to help you learn how to do the math in your head—when you get the idea, you will not need to memorize every little step. In fact, with five examples in this chapter and 25 more in an appendix, you will be able to practice enough to easily memorize the process.

For reference, the steps in the process are as follows:

Step 1 Identify the structure of the IP address.

Step 2 Create the chart that will be used in Steps 3 and 4.

Step 3 Derive the subnet number and the first valid IP address.

Step 4 Derive the broadcast address and the last valid IP address.

In this section, you will see two different examples worked out in great detail and three examples worked out with less detail. With these five examples, plus the extra practice in Appendix C, you will be able to master the process of answering the previous questions about any IP address. If you already can answer those questions about the five examples, you know plenty about IP subnetting for passing the CCNA exam! Here are the examples:, mask, mask, mask, mask, mask

If you can confidently answer all four questions for all five example address/mask pairs, you might want to skip the rest of the IP addressing section. If you look at the examples and think that you simply need more practice, turn to Appendix C, which has 25 additional examples completely worked out. If you want to see how I got the answers to these five, along with the process to do it quickly without needing a binary-to-decimal conversion chart, read on! By the way, you can check your work against Table 6-13, which summarizes the results of all the steps for each of the five sample addresses and masks.

Table 6-13 Five Addresses/Masks, with IP Addressing Information Explained


Number of network bits






Number of host bits






Number of subnet bits






Number of hosts per subnet

216 - 2

28 - 2

28 - 2

210 - 2

25 - 2

Number of subnets

28 - 2

28 - 2


26 - 2

23 - 2

Subnet number

First valid IP address

Broadcast address

Last valid IP address

Range of valid IP addresses

0 0


  • Pansy Took-Took
    What is sunet mask and broadcast address of
    2 years ago

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