Answers: How Many Hosts/Subnets Speed Practice 3

 In 200-301 V1 Ch15: Subnet Design, 200-301 V1 Part 4: IPv4 Addressing, CCENT-OLD, IPv4 Mask Drills

Today’s post answers the five questions in a previously posted exercise. What’s it about? Someone came before us at most every company and made up an IP addressing plan. These exams test our ability to look at that plan and decipher what the plan does, including answering this question:

  • “How many subnets does this plan create?”
  • “Host many hosts per subnet”

Check out the earlier post for the questions, and this post for the answers! Enjoy!

How to Find the Answers

The process to find the answers, given one key assumption, requires only basic math. Like many subnetting questions, figuring out what math to use can be the difficult part.

With the assumption of a single mask used throughout an entire classful network, you can predict the number of subnets. Without that assumption, you cannot. The problem statement told us that the design used a single mask, so that problem was conveniently solved.

The rest of the math revolves around analyzing the mask and the class of network to determine the number of network, subnet, and host bits. Figure 1 shows the general concept. You use class rules to determine the size of the network part (8, 16, or 24 bits). With a little addition and subtraction and the prefix-style mask you can calculate the number of subnet and host bits. From there, just use these formulas:

  • For the number of subnets: 2^S
  • For the number of hosts per subnet: 2^H – 2

Figure 1 shows the main parts of the logic.

Figure 1: Conceptual View of a Subnet Mask, with N, S, P, and H Bits

Answers to These Problems

Table 1 lists the analysis of the five problems, listing the number of Network (N), Subnet (S), and Host (H) bits in each case. Following that, Table 2 lists the literal answers, taking the S and H values in each case to calculate the number of allowed subnets and hosts.

Table 1: Sizes of N, S, and H

Class

Network Bits (N)

Subnet Bits (S = P – N)

Host Bits (32 – P)

1

A

8

7

17

2

B

16

2

14

3

C

24

3

5

4

C

24

5

3

5

B

16

7

9

Table 2: Answers, Numbers of Subnets and Hosts per Subnet

 

Number of Subnets

Number of Hosts

1

2^7 = 128

2^17 – 2 = 131,070

2

2^2 = 4

2^14 – 2 = 16,382

3

2^3 = 8

2^5 – 2 = 30

4

2^5 = 32

2^3 – 2 = 6

5

2^7 = 128

2^9 – 2 = 510

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Kevin Titus

Thanks again, this has helped me understand how to determine the # of networks. I could never wrap my head around it up until reading this.

Best,
Kevin Titus

lyjo

Sure thing, Kevin! Glad to help.
Wendell

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