Subnetting Questions and Answers

Subnetting Question 1

On a local subnet with the 255.255.224.0 subnet mask, what is the maximum number of IP addresses that can be issued to hosts? 

1. 8190
2. 4094
3. 16382
4. 2048

Answer

A. 8190

Solution

The given subnet mask is 255.255.224.0. The third octet has 224, which means three consecutive ones that are:

So the number of zeros = 13

The maximum number of IP addresses that hosts can issue is 2¹³ – 2, which is 8190.

Therefore option A is correct.

Subnetting Question 2

Suppose an organization want 6 subnets, and each subnet has at least 22 hosts. What subnet mask should the organization use to fulfil the above condition?

1. 255.255.255.128
2. 255.255.255.224
3. 255.255.255.240
4. 255.255.255.255

Answer

B. 255.255.255.224

Solution

We see that the given subnet mask is of a class C IP address. So the subnetting is done in the last octet.

 If the organization want 6 subnets, so the 3 bits are required that is 2³ = 8.

The remaining bits in the last octet = 8 – 3 = 5.

So the number of hosts = 2⁵ – 2 = 30

But the organization won't only 22 host, which is less than 30. Hence the organization's condition is fulfilled.

Therefore the subnet mask is:

Thus option B is correct.

Subnetting Question 3

Consider a subnet mask of 255.255.255.128 with an IP address of 200.49.89.99. The network wants to manually create subnets or define them as zero subnets and DBA subnets.

Consider the following assertions:

1. The subnet ID for the zero subnets is 200.49.89.0.
2. The zero subnet's last host is 200.49.89.126.
3. The DBA subnet's first host is 200.49.89.127.
4. 200.49.89.255 is the DBA of the DBA subnet.

Which of the following assertion is true?

1. Statement I, II, and III are true.
2. Statement I, II, and IV are true.
3. Statement I and IV are not true.
4. None of these

Answer

B. Statement I, II, and IV are true

Solution

Given, IP address = 200.49.89.99

Subnet mask = 255.255.255.128(in decimal) = 255.255.255.10000000(in binary)

As the given IP is of class C, so the total subnets = 2¹ = 2

Here, we have explicitly configured the zero subnet and DBA.

Statement I:

So, the zero subnet ID is 

= 200.49.89.0

Statement II:

Last host of the zero subnets.

= 200.49.89.126

Statement III:

Now, IP address = 200.49.89.0 1100011

Now, we need to calculate the first host of the DBA subnet.

So, the required IP is :

= 200.49.89.129

Statement IV:

Here, we need to find the DBA of the DBA subnet.

So, the required IP is:

= 200.49.89.255

Hence, option B is the correct answer.

Subnetting Question 4

The subnet mask of a subnetwork is 255.255.224.0, IP address of a host in this subnet is 162.25.98.83. What is the number of the subnet in that network?

1. 2⁵
2. 2⁸
3. 8
4. Not possible to determine

Answer

B. 2⁸

Solution

Given that,

162.25.98.83 is the IP address of class B, with 16 bits for network ID, the rest 16 bits for host & subnet ID.
Subnet mask 255.255.224.0 =255.255.11100000.000000000
So 3 bits for subnet ID.
So the number of the subnet in the network is 2³= 8

Therefore option B is correct.

Subnetting Question 5

If the subnet mask is 255.255.255.224, the number of subnets is X, and the number of hosts in each subnet is Y in Class C, what is X+Y?

1. 30
2. 38
3. 32
4. 25

Solution

255.255.255.0 is the default subnet mask for Class C.

We steal bits from the host while subnetting.

As a result, the subnet mask is 255.255.224, or 11111111.11111111.11111111.11100000.

The default mask is 11111111.11111111.11111111.00000000. Therefore, we shall borrow three bits from the host part.

The total amount of subnet bits is three.

Also, the total number of host bits is equal to 5.

As a result, subnet number (X) = 2^{subnet bits}= 8

And each subnet's number of hosts (Y) = 2^{host bits} - 2 = 30

X=8 and Y=30

X + Y = 38

Hence, the option B is correct.

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