# CCNA – Subnetting

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

Question 1

Explanation

310 hosts < 512 = 2^{9} -> 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 = 2^{7} -> 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 2^{3} – 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 2^{6} = 64 subnets but only 2^{2} – 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 2^{4} = 16 subnets but only 2^{4} – 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 2^{3} = 8 subnets and 2^{5} – 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 = 2^{5} 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.

Question 3

Explanation:

… 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.

6 > 5 hm, we need just 3 hosts, so I would calculate with 3 and not 5.

Yeah I cant see the questions either….

Hi

In an interview i got a questio that 170.16.8.0/12

Determine network id , fhid, lhid and broadcast.

Please answer this

Address: 172.16.8.0 10101100.0001 0000.00001000.00000000

Netmask: 255.240.0.0 = 12 11111111.1111 0000.00000000.00000000

Wildcard: 0.15.255.255 00000000.0000 1111.11111111.11111111

=>

Network: 172.16.0.0/12 10101100.0001 0000.00000000.00000000 (Class B)

Broadcast: 172.31.255.255 10101100.0001 1111.11111111.11111111

HostMin: 172.16.0.1 10101100.0001 0000.00000000.00000001

HostMax: 172.31.255.254 10101100.0001 1111.11111111.11111110

Hosts/Net: 1048574

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I did not see the questions , just only show the explanations of the questions.. So what is the problem??? why the questions are not shown????

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Please send me dumb questions of ccna new version syllabus

can any one explain Q10 plz coz the explanation provided is not enough.

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Need help#

Suppose You have a big company of 63 hosts per network. You are the System Administrator of your company. You have to plan the addressing scheme. Your are given IP 172.72.157.253 And your Dept. (Engtneering) uses Subnet N 399.

Q. What will be the Subnet bit?