[SOLVED] CIDR help

[ThatsAJokeK]

I am looking for someone that can help me understand how to get to the answer, the correct answer they gave was /16 and my guess was /20 my thought process was that /22 has 1024 host and /21 has 2048 host 1024+2048=3072 so /20 having 4096 host is how i got my answer


Based on networks 10.8.16.0/22 and 10.8.31.0/21, which of the following is the BEST summarized CIDR notation?
A. 10.8.0.0/16
B. 10.8.0.0/19
C. 10.8.0.0/20
D. 10.0.0.0/24

 

[ThatsAJokeK]

I've looked at other sites that have this question but it seems a lot of the commenters have different opinions on if /16 was correct or if /19 would be a better fit

 

[kanewolf]

I am looking for someone that can help me understand how to get to the answer, the correct answer they gave was /16 and my guess was /20 my thought process was that /22 has 1024 host and /21 has 2048 host 1024+2048=3072 so /20 having 4096 host is how i got my answer


Based on networks 10.8.16.0/22 and 10.8.31.0/21, which of the following is the BEST summarized CIDR notation?
A. 10.8.0.0/16
B. 10.8.0.0/19
C. 10.8.0.0/20
D. 10.0.0.0/24

A /19 would cover all the addresses desired. Plug the values into a CIDR calc -- http://www.subnet-calculator.com/cidr.php and you can see the results.
The /19 also allows additional blocks to be handed out independently compared to the /16. I would say a /19 is the BEST answer.

 

[bill001g]

Did you typo the ip addresses.

10.8.31.0 is not a valid /21 boundary . The third position must be divisable by 8 since it has has 2048 addresses. This means if you 10.8.31.0 is inside the block the network address has to be 10.8.16.0. Which overlaps the /22.

The 10.8.16.0/22 is ok. It runs from 10.8.16.0-10.8.19.255

Now lets say you typo and it should be 10.8.32.0/21. This is a valid boundary and has ip 10.8.32.0-10.8.39.255.

In this case the only answer that will work is 10.8.0.0/16

10.8.0.0/19 has addresses 10.8.0.0-10.8.31.255. It needs to have 32*256 to get 8k of IP. So it does not cover all the way to 10.8.39.255.

You would have to use a /18 which goes all the way to 10.8.63.255

Part of the trick is all the unused IP addresses.


.......well I thought about this and my math is bad. the 10.8.31.0/21 has a network IP of 10.8.24.0 so does not overlap. It runs form 10.8.24.0-10.8.31.255. So a /19 is the correct answer.

 

[ThatsAJokeK]

A /19 would cover all the addresses desired. Plug the values into a CIDR calc -- http://www.subnet-calculator.com/cidr.php and you can see the results.
The /19 also allows additional blocks to be handed out independently compared to the /16. I would say a /19 is the BEST answer.

anyway you could explain your thought process cause I'm pretty sure I have the wrong mindset for the question cause im still thinking if 19 covers all the desired address wouldn't /20 do the same and have less un-used ips?

 

[kanewolf]

anyway you could explain your thought process cause I'm pretty sure I have the wrong mindset for the question cause im still thinking if 19 covers all the desired address wouldn't /20 do the same and have less un-used ips?

The issue with the question as proposed is the 10.8.31.0/21 requirement. How that is interpreted since .31 is not a valid boundary for a /21 CIDR. If you assume that is .24.0/21 (which includes up to .31.255) then my /19 would be correct. To be a /20 the starting address would have to be 10.8.16.0 rather than 10.8.0.0 as shown in the answers.

 

[ThatsAJokeK]

The issue with the question as proposed is the 10.8.31.0/21 requirement. How that is interpreted since .31 is not a valid boundary for a /21 CIDR. If you assume that is .24.0/21 (which includes up to .31.255) then my /19 would be correct. To be a /20 the starting address would have to be 10.8.16.0 rather than 10.8.0.0 as shown in the answers.

I guess now I'm wondering why is 10.8.31.0 not a valid boundary for a /21 CIDR I'm kinda new to all this so I sure I'm missing some key information that would help me understand this easier also thank you for helping

 

[bill001g]

You need to look at some of the sample of how to find the network address given a ip address and subnet mask. There are a couple ways to do this. One of the more common ways is to write it out in binary and then take the mask length number of bits. So for a /19 you would take the first 19 bits and then convert it back to decimal. This is commonly the way that is taught because you don't have to memorize tables.

The method I use is to memorize the table of subnets to "/". So you know for example a /21 is 255.255.248.0. Now the trick is you subtract 248 from 256. This gives you 8.... i will ignore that the last part octtet is zero. What you then do is divide the ip by this number. You get 31 divided by 8 which is 3.????? You don't need to actually get the detailed. You then take the 3 and multiply it back by 8 and get 24. This is the network address.

The second method is much much faster but you have to know the subnet table and be able to make the calculations quickly. I have been doing this over 20yrs so I do it without thinking....and make stupid mistakes like thinking a /21 was 240.