# Subnetting Query (cbt)

Guile101
Posts:

**8**Registered Users ■□□□□□□□□□
in CCNA & CCENT

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?

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?

0

## Comments

199MemberIt'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.

1,103MemberThis helped me a bit.

8Registered Users ■□□□□□□□□□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,

1,103MemberThe 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.

191MemberWhat 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

8Registered Users ■□□□□□□□□□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.

8Registered Users ■□□□□□□□□□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?

8Registered Users ■□□□□□□□□□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

199MemberGo 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

389MemberAnother 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.

11111111.11100000with 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.

389Member8Registered Users ■□□□□□□□□□444MemberWhich 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.

191MemberFor 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

444MemberYes 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.

1,218MemberFind 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.

389MemberWhat is the last valid host on the subnetwork 10.55.96.0/20?

(1) 10.55.96.0 =

00001010.00110111.01100000.00000000(2) /20 =

11111111.11111111.11110000.00000000AND (1) & (2)

00001010.00110111.0110|0000.0000000011111111.11111111.1111|0000.00000000============================00001010.00110111.0110|00000000 <=== Network ID0000.==================================

00001010.00110111.0110|00000000 = 10.55.96.0 (Network ID)0000.Now to find this network's broadcast address, simply inverts all the 0's

the red divider(after|) to 1's00001010.00110111.0110|1111.11111111= 10.55.111.255 (Broadcast address)We know that broadcast address is the

and it cannot be used as a host address, but the one address justlast address in a networkbeforeit can be used as ahost address(hence, terms aslast valid host address). We also know that, in this case, thelast 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.

389Memberok, 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 =

11111111.11111111.11111100.00000000AND (1) & (2)

10101100.00011101.111100|11.0010001111111111.11111111.111111|00.00000000============================10101100.00011101.111100|00.00000000 <=== Network ID==================================

10101100.00011101.= 172.29.240.0 (Network ID)111100|00.00000000Again, 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.= 172.29.240.1 (First valid host address)111100|00.00000001Therefore 10.55.240.1 is the answer.