CCNA – Subnetting

Note: If you are not sure about Subnetting, please read our Subnetting Tutorial – Subnetting Made Easy.

Question 1

Explanation

310 hosts < 512 = 2⁹ -> We need a subnet mask of 9 bits 0 -> 1111 1111.1111 1111.1111 1110.0000 0000 -> 255.255.254.0

Question 2

Explanation

Network A needs 120 hosts < 128 = 2⁷ -> Need a subnet mask of 7 bit 0s -> “/25″.

Because the ip subnet-zero command is used, network 172.16.3.0/30 can be used.

Answer E “Link A – 172.16.3.40/30″ is not correct because this subnet belongs to MARKETING subnet (172.16.3.32/27).
Answer F “Link A – 172.16.3.112/30″ is not correct because this subnet belongs to ADMIN subnet (172.16.3.96/27).

Question 3

Explanation

First we should observe the answers, they are all 192.168.252.x so they are in class C with a default subnet mask of 255.255.255.0. We need three web servers (three hosts) in a subnet so we need 3 bits 0 (using the formula 2³ – 2 = 6 > 5 hosts) so the last octet of our subnet mask should be 1111 1000 -> 248.

Question 4

Explanation

A is not correct because it is a wildcard mask (not subnet mask).

This question is a bit unclear but we can suppose we have to begin with default subnet mask and “subnet” it. And the default subnet mask here should be class C: 255.255.255.0

For answer B: 252 = 1111 1100 -> with this subnet mask we can subnet up to 2⁶ = 64 subnets but only 2² – 2 = 2 hosts per subnet -> B is not correct.

C is not correct because it is the default subnet mask of class C and that means we don’t “subnet” it.

For answer E: 240 = 1111 0000 -> There are 2⁴ = 16 subnets but only 2⁴ – 2 = 14 hosts per subnet < 26 hosts -> E does not satisfy the second requirement (of 26 hosts per subnet).

For answer D: 224 = 1110 0000 -> There are 2³ = 8 subnets and 2⁵ – 2 = 30 hosts > 26 hosts -> This is the correct answer.

Note: The number “5” in ” with each LAN containing 5 to 26 hosts” is just used to trick you and it does not have any effect on our answer.

Question 5

Question 6

Explanation

Router A receives 3 subnets: 172.16.64.0/18, 172.16.32.0/24 and 172.16.128.0/18.

All these 3 subnets have the same form of 172.16.x.x so our summarized subnet must be also in that form -> Only A, B or C is correct.

The smallest subnet mask of these 3 subnets is /18 so our summarized subnet must also have its subnet mask equal or smaller than /18.

-> Only answer A has these 2 conditions -> A is correct.

Question 7

Explanation

Each subnet has 30 hosts < 32 = 2⁵ so we need a subnet mask which has at least 5 bit 0s -> /27. Also the question requires the maximum number of subnets (which minimum the number of hosts-per-subnet) so /27 is the best choice -> A is correct.

Question 8

Question 9

Explanation

We need 113 point-to-point links which equal to 113 sub-networks < 128 so we need to borrow 7 bits (because 2^7 = 128).

The network used for point-to-point connection should be /30.
So our initial network should be 30 – 7 = 23.

So 10.10.0.0/23 is the correct answer.

You can understand it more clearly when writing it in binary form:

/23 = 1111 1111.1111 1110.0000 0000
/30 = 1111 1111.1111 1111.1111 1100 (borrow 7 bits)

Question 10

Explanation

Increment: 16 (of the third octet)
Network address: 172.16.16.0

-> A is correct.