Subnetting Query (cbt)

Hi,

I have been learning subnetting through CBT nuggets and I understand the method clearly to find out how many hosts and networks, However I have tried to do some other subnetting question examples and they are not really adding up. The problem is the questions are being asked in a totally different way than the method in the cbt videos so imn not sure where to begin troubleshooting them but I know that it cant be that difficult, just that the question is asked completely different.

Can anyone advise or help??

Thanks in advance

What is the last valid host on the subnetwork 10.55.96.0/20?

What is the first valid host on the subnetwork that the node 172.18.221.7 255.255.254.0 belongs to?

What is the first valid host on the subnetwork that the node 172.29.243.35/22 belongs to?

Find more posts tagged with

Save $250 on 2025 certification boot camps from Infosec!

Book now with code EOY2025

Button

Comments

okplaya

Could you explain the steps you've taken to attempt to answer these, and where you get confused?

It's much better if you let us know where you get confused and we can help from there. If I post the breakdown of how I solve this it may just confuse you further. Waiting on your response.

ehnde

http://www.techexams.net/forums/ccna-ccent/38772-subnetting-made-easy.html

This helped me a bit.

Guile101

okplaya wrote: »

Could you explain the steps you've taken to attempt to answer these, and where you get confused?

It's much better if you let us know where you get confused and we can help from there. If I post the breakdown of how I solve this it may just confuse you further. Waiting on your response.

Hi,

The problem is that im not too sure how to tackle these questions now after learning the method in CBT. The questions in CBT are breaking the network into host or networks or reverse engineering but the examples below are looking for valid hosts etc.

Im new to subnetting so the method from the video seemed fine to me but when I tried to look at some other example questions i cant see how I can use the cbt method to figure them out.

I hope this makes sense

Thanks,

ehnde

Those CBT nugget videos are the long way of doing subnetting problems. They help you wrap your mind around the concepts, but you need to be fast. Did you look at that thread I linked? Faster and easier method, won't take but 10 minutes to learn and it compliments your newfound knowledge from the CBT nugget videos.

The only way to get past this is practice. subnettingquestions.com - Free Subnetting Questions and Answers Randomly Generated Online Don't get discouraged when you are wrong! Practice is the key. I started studying for the CCNA exam in December and finished my studies this March. I probably racked up 15 hours of subnetting practice during that time. That doesn't count the time learning methods of subnetting.

Once you have enough practice the method doesn't matter. You just know the answer.

Futura

It does take a while to sink in, but once it does is easy as pie.

What is the last valid host on the subnetwork 10.55.96.0/20

Class A subnet is 8 bits,

this is 20 bits so looks like this 11111111.11111111.11110000.00000000

look at the third octet, interesting one? Increment is 16?

so, bear in mind the third octet, increments of 16:
16.0 network id - 31.255 broadcast
32.0 47.255 broadcast
48.0
64.0
80.0
96.0 network 111.255 broadcast
112.0

Network is the first, broadcast is the last, anything in-between is fair game! so the last valid address is 111.254

Hope this helps

Guile101

Futura wrote: »

It does take a while to sink in, but once it does is easy as pie.

What is the last valid host on the subnetwork 10.55.96.0/20

Class A subnet is 8 bits,

this is 20 bits so looks like this 11111111.11111111.11110000.00000000

look at the third octet, interesting one? Increment is 16?

so, bear in mind the third octet, increments of 16:
16.0 network id - 31.255 broadcast
32.0 47.255 broadcast
48.0
64.0
80.0
96.0 network 111.255 broadcast
112.0

Network is the first, broadcast is the last, anything in-between is fair game! so the last valid address is 111.254

Hope this helps

Thanks, This makes perfect sense looking at it and Im off know to practice, practice, practice.

I think I have the general understanding of finding the answer but Im not quite grasping the different type of subnetting questions YET.

Hopefully it will all fall into place soon.

Thanks for everyone's help in this thread.

Guile101

Futura wrote: »

It does take a while to sink in, but once it does is easy as pie.

What is the last valid host on the subnetwork 10.55.96.0/20

Class A subnet is 8 bits,

this is 20 bits so looks like this 11111111.11111111.11110000.00000000

look at the third octet, interesting one? Increment is 16?

so, bear in mind the third octet, increments of 16:
16.0 network id - 31.255 broadcast
32.0 47.255 broadcast
48.0
64.0
80.0
96.0 network 111.255 broadcast
112.0

Network is the first, broadcast is the last, anything in-between is fair game! so the last valid address is 111.254

Hope this helps

Hi,

If you have some time could you answer these questions for me as well please , Kind of stuck on them at the moment. Thanks

1. You are designing a subnet mask for the 172.21.0.0 network. You want 1100 subnets with up to 18 hosts on each subnet. What subnet mask should you use?

2. What is the broadcast address of the network 172.23.6.64/27?

Guile101

Hi again,

Really struggling with sub netting, the example explained above for me makes sense for the last valid host on the subnetwork and any questions I get that are worded like that on sub netting questions I can answer. Problem is when the question changes or is asked another way I'm completely stuck and don't know where to start, guess what I'm striving for is a generic way I can tackle any sub netting questions instead of one kind. I want to get practicing on questions.net but keep getting stuck.

Any help would be greatly appreciated.

Thx

okplaya

Guile101 wrote: »

Hi again,

Really struggling with sub netting, the example explained above for me makes sense for the last valid host on the subnetwork and any questions I get that are worded like that on sub netting questions I can answer. Problem is when the question changes or is asked another way I'm completely stuck and don't know where to start, guess what I'm striving for is a generic way I can tackle any sub netting questions instead of one kind. I want to get practicing on questions.net but keep getting stuck.

Any help would be greatly appreciated.

Thx

Go back to the basics like in the cbt nuggets. You can learn a generic way, but if you don't understand the underlying concepts, it will likely backfire on you.

1. You are designing a subnet mask for the 172.21.0.0 network. You want 1100 subnets with up to 18 hosts on each subnet. What subnet mask should you use?

Well, we know that this is a class B address right (172.16.00 - 172.16.31.255.255). We know that Class A has 8 network bits, Class B has 16, and Class C has 24 right? So with that information would you be able to figure out the number of bits in the network portion of the address is 16. So we have (N = Network bits, H = host bits) nnnnnnnn nnnnnnnn hhhhhhhh hhhhhhhh. We are allowed to borrow from the host (h) portion to subnet. So from there you can use the CBT method to answer your question.

HTH

chopsticks

okplaya wrote: »

1. You are designing a subnet mask for the 172.21.0.0 network. You want 1100 subnets with up to 18 hosts on each subnet. What subnet mask should you use?

Well, we know that this is a class B address right (172.16.00 - 172.16.31.255.255). We know that Class A has 8 network bits, Class B has 16, and Class C has 24 right? So with that information would you be able to figure out the number of bits in the network portion of the address is 16. So we have (N = Network bits, H = host bits) nnnnnnnn nnnnnnnn hhhhhhhh hhhhhhhh. We are allowed to borrow from the host (h) portion to subnet. So from there you can use the CBT method to answer your question.

HTH

Another practice opportunity for me

172.21.0.0 is a class B address. And class B address will have its default mask to be 255.255.0.0 or, 11111111.11111111.00000000.00000000

To have 1100 subnets, we need to borrow 11 bits from the host portion, because 2^11 = 2048, which is more than enough to cover 1100 subnets (we cannot just borrow 10 bits from the host portion because 2^10 is only equal to 1024, which is lesser than 1100 subnets required).

The remaining host bits will be 5 0s, so (2^5)-2 will give us 30 hosts on each subnet, which is more than enough to cover 18 hosts per subnet.

Therefore, the new subnet mask in binary would look like this

11111111.11111111.11111111.11100000

with the bold to indicate that they are the borrowed bits, and blue is the total number of bits use to constitute a subnet mask, so the new subnet mask in numberical representation is 255.255.255.224 or /27.

Please feel free to correct my mistakes.

chopsticks

My calculated answers are in blue

Guile101 wrote: »

What is the last valid host on the subnetwork 10.55.96.0/20?

The last valid host is 10.55.111.254

What is the first valid host on the subnetwork that the node 172.18.221.7 255.255.254.0 belongs to?

The first valid host is 172.18.220.1

What is the first valid host on the subnetwork that the node 172.29.243.35/22 belongs to?

The first valid host is 172.29.240.1

Guile101

How did you calculate them??

Eildor

There are a number of subnetting questions you need to be able to answer -- don't try and learn how to answer them all at once. First, learn how to answer the following type of question:

Which subnet does host 172.31.193.1/21 belong to?

This is how to work it out, and I'd recommend writing out the entire thing in binary until you understand what is happening.

Step 1 - Write out the entire address, along with the subnet mask, in binary.

10101100.00011111.11000001.00000001
11111111.11111111.11111000.00000000

Step 2 - "And" the whole thing.

If you don't understand what to do, let me know.

Futura

I love the questions that say 'you need so many subnet's, what mask to use?'

For this know your powers

2^6 64 & 2^10 = 1024 are a good quick starting point

So example. You have been asked to design a network using the 172 Class B address and you need 900 neworks.

So that must be 2^10 = 1024 networks cause 2^9 = 512 and is too low.

For the answer you know you need 10 subnet bits plus the class b address bits. 10 + 16 = Subnet Mask

nnnnnnnn.nnnnnnnn.ssssssss.sshhhhhh

Simples

Eildor

Futura wrote: »

I love the questions that say 'you need so many subnet's, what mask to use?'

For this know your powers

2^6 64 & 2^10 = 1024 are a good quick starting point

So example. You have been asked to design a network using the 172 Class B address and you need 900 neworks.

So that must be 2^10 = 1024 networks cause 2^9 = 512 and is too low.

For the answer you know you need 10 subnet bits plus the class b address bits. 10 + 16 = Subnet Mask

nnnnnnnn.nnnnnnnn.ssssssss.sshhhhhh

Simples

Yes my meerkat loving friend

I think that first learning how to work out the subnet mask required for x amount of hosts, makes it easier to then learn how to work out the subnet mask required for x amount of subnets. As you know, when you are working out the subnet mask for x amount of hosts, you don't have to worry about the class of the network, so it makes it just that bit easier to grasp.

brad-

Guile101 wrote: »

The questions in CBT are breaking the network into host or networks or reverse engineering but the examples below are looking for valid hosts etc.

Looking for valid hosts is what you would do after you do your subnetting.

Find your increment, then you can write out the network ranges. Finding the valid hosts is the easy part that you just exclude the first and last address in the network range for the network ID and broadcast. In looking for valid addresses, always exclude the first and last address...your valid addresses are inbetween that.

chopsticks

Guile101 wrote: »

How did you calculate them??

What is the last valid host on the subnetwork 10.55.96.0/20?

(1) 10.55.96.0 = 00001010.00110111.01100000.00000000
(2) /20 = 11111111.11111111.11110000.00000000

AND (1) & (2)

00001010.00110111.0110|0000.00000000
11111111.11111111.1111|0000.00000000
============================
00001010.00110111.0110|0000.00000000 <=== Network ID
==================================

00001010.00110111.0110|0000.00000000 = 10.55.96.0 (Network ID)

Now to find this network's broadcast address, simply inverts all the 0's after the red divider(|) to 1's

00001010.00110111.0110|1111.11111111 = 10.55.111.255 (Broadcast address)

We know that broadcast address is the last address in a network and it cannot be used as a host address, but the one address just before it can be used as a host address (hence, terms as last valid host address). We also know that, in this case, the last address (broadcast address) is 255, and the number before it is 254.

Therefore 10.55.111.254 is the answer.

You can use this way to try out the rest of the questions and feel free to discuss again.

chopsticks

Guile101 wrote: »

How did you calculate them??

ok, maybe one example is not adequate enough, let us try the last question?

++++++++++++++++++++++++++++++++++++

What is the first valid host on the subnetwork that the node 172.29.243.35/22 belongs to?

++++++++++++++++++++++++++++++++++++

(1) 172.29.243.35 = 10101100.00011101.11110011.00100011
(2) /22 = 11111111.11111111.11111100.00000000

AND (1) & (2)

10101100.00011101.111100|11.00100011
11111111.11111111.111111|00.00000000
============================
10101100.00011101.111100|00.00000000 <=== Network ID
==================================

10101100.00011101.111100|00.00000000 = 172.29.240.0 (Network ID)

Again, we know that the first address in this network cannot be used as host address, because it is used to represent the network ID. We also know that the next address following it can be used as host address and it is called the first valid host address:

10101100.00011101.111100|00.00000001 = 172.29.240.1 (First valid host address)

Therefore 10.55.240.1 is the answer.