Subnetting Questions – Tackling Three Types of Networking Solutions

subnetting questionsThe good news is that when studying IP subnetting there are only three broad types of questions that seem to crop up. The first type will ask for a network id, a broadcast address or perhaps the first and last valid IP addresses for a given address. This is a very common type question and fortunately there are quick and easy ways to answer them. Another type of question is how many subnets or hosts can be derived from a given network or mask. Again, this is straightforward with a little practice. The third type is more difficult, as this type of question is concerned with the student’s knowledge and ability to transfer that knowledge to a design scenario. This type of question may or may not involve variable length subnet masking. This article will cover all three questions.

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An Introduction to Basic Subnetting

What is the Network ID, Broadcast Address, and first & last valid IP on the subnetwork that the host 192.168.1.15/26 belongs to?

  • Firstly, work out the network block size. The mask is /26 and it is in the fourth octet (255.255.255.192) so (32-26) = 6 then 2^6=64 (alternatively subtract 256-192 =64)
  • If the network block is 64 then our first network, counting up from zero will be 192.168.1.0 – 192.168.1.63; therefore our Network ID is 192.168.1.0 and the broadcast address is 192.168.1.63
  • The network block is 64 so 64 -2 (the network ID and Broadcast aren’t valid hosts) gives us 62 hosts, the first is 192.168.1.1 the last 192.168.1.2

Test A

1.      What is the Network ID, Broadcast Address, and first & last valid IP on the subnetwork that the host 172.30.11.15/26 belong to?

 2.      What is the Network ID, Broadcast Address, and first & last valid IP on the subnetwork that the host 172.19.0.0 255.255.254.0 belong to?

 3.      What is the Network ID, Broadcast Address, and first & last valid IP on the subnetwork that the host 192.168.1.1/30 belong to?

Now, let’s take a look at the second type of question, the one that concerns itself with subnets and hosts. Here is a typical example

How many subnets and hosts per subnet can you get from the network 172.168.21.0 255.255.255.224?

  • First look to the network mask, it has 224 in the fourth octet, so it is borrowing bits from the hosts to extend its reach. So how many bits has it borrowed? Looking at the binary to decimal overlay, it is three bits.

128  64 32  16 8  4  2  1

1    1    1   0  0  0  0  0

  • So 3 bits have been commandeered for subnets and five bits remaining for hosts, which is 2^3 = 8 subnets and 2^5 = 32 hosts

Test B

1.      How many subnets and hosts per subnet can you get from the network 192.161.204.0/30?

2.      How many subnets and hosts per subnet can you get from the network 192.16.21.0 255.255.255.128?

3.      How many subnets and hosts per subnet can you get from the network 172.27.0.0 255.255.255.224?

The other type of question is scenario based where a network will require to be addressed using a suitable IP plan, here is a typical example and solution.

You have been asked to design an address plan that will accommodate the 172.16.0.0 network. Your organization requires 1000 subnets, with at least 50 hosts per subnet. What subnet mask should you use?

  • The clue here is the question asks for singular subnet mask, so it will not involve VLSM there will be 1000 subnets all with 50 hosts
  • The address given is a class B,  bits will be borrowed from the third and possibly fourth octet to create our 1000 subnets
  • Calculate what power of 2 which will return greater than 1000, it is 10,

2^10 = 1024

(Remembering the powers of two, comes in handy),

1   2  3   4    5    6    7      8      9      10      11
2,  4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048 …
  • The subnet mask is therefore, 255.255.255.192 or /26 which means there are 6 bits remaining for hosts 2^6 = 64

64 -2 = 62 valid hosts per subnet, which meets the requirements.

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Test C

1.      You have been asked to design an address plan that will accommodate the 172.16.0.0 network. Your organization requires 400 subnets, with at least 70 hosts per subnet. What subnet mask should you use?

 2.      Find which subnet the host 10.154.124.201 255.255.252.0 belongs?

3.      Enter the last valid host on the network that the host 172.19.197.206/30 is a part off

4.      You have been asked to design an address plan that will accommodate the 172.16.0.0 network. Your organization requires 1500 subnets, with at least 50 hosts per subnet. What subnet mask should you use?

5.      Enter the broadcast address for the network 172.17.168.40 255.255.255.248

6.      What is the shorthand corresponding to a subnet mask of 255.255.128.0?

7.      What is the maximum number of valid subnets and hosts per subnet that you can get from the network 172.28.0.0/22

8.      Which subnet does the host 192.168.137.249 belong?

Take a course at Udemy, and answer all 8 of these questions.

Answers Test A

1Network ID 172.30.11.0Broadcast Address 172.30.11.63First & Last 172.30.11.1 – 62
2Network ID 192.168.230.144Broadcast Address 192.168.230.151First & Last 192.168.230.145 – 150
3Network ID 192.168.1.0/30Broadcast Address 192.168.1.3First & Last 192.168.1.1 – 2

Test B

164 subnets2 hosts,
2128 subnets510 hosts
32048 Subnets30 hosts

Test C

1/25 or 255.255.255.128
210.154.124.0
3172.19.197.206
4/27 or 255.255.255.224
5172.17.168.47
6/17
71024 hosts 62 hosts
8192.168.137.192