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C
00001010.00001000.00010000.00000000 (10.8.16.0)
00001010.00001000.00011111.00000000 (10.8.31.0)
The last bit that they have in common is the 3rd octet 4th bit. (bit 20)
It would normally be a /21 but the best choice of the ones given is /20
Unfortuntely no! /20 will give you 10.8.15.255 (count your own bits- there are 16+3=19) as the broadcast, but /19 will cover 10.8.31.255- so /19 is the BEST at this particular logic. But I guess our BEST involves VLMS( Variable Length Subnet Masking) where we choose sufficient but not excessive subnet masking and following that logic- it should be /19. BUT, CompTIA disregards it and focuses on full octet usage, which since .16.0 and .31.0 in example showing such- means /21. That is the only logical explanation I have!
TT is 99.9999% right. The answer is "C" but to do the summarization you have to count how many bits the common pattern has and the common pattern for this question is:
00001010.00001000.0001 (20)
At first I thought KC had it right, but then I realized I had neglected to include the fact that the 20th bit was also the same for both addresses, which make 20 bits in common and thus /20 for the summarization. I agree with TT and TeflonJimmy.
Definitely A is correct
CIDR Notation is /20 but Question is asking about summarized (Short) CIDR which is /16.
IP Address: 10.8.0.0/16
Network Address: 10.8.0.0
Usable Host IP Range: 10.8.0.1 - 10.8.15.254
Broadcast Address: 10.8.15.255
Total Number of Hosts: 4,096
Number of Usable Hosts: 4,094
Subnet Mask: 255.255.240.0
Binary Subnet Mask: 11111111.11111111.11110000.00000000
CIDR Notation: /20
IP Type: Private
*** Short: 10.8.0.0/16 /20 ****
Let me give my way of counting subnets.
16 bit = 65 aprox
64 × 4 = 255 aprox hops of IP
Don't know why but when I multiply with 4 I get aporx address of subnets with reminder of -2 to score
based on multiple subnet calculators I have utilized (here is a link to one of them: https://www.subnet-calculator.org/supernets.php) the answer should be C 10.8.16.0/20
/16 is the right answer watch IP Subnetting from CIDR Notations on youtube
00001010.00001000.00010000.00000000 (10.8.16.0) is a /22
00001010.00001000.00011111.00000000 (10.8.31.0) is a /21 so you take the common subnet bit from each ip according to their CIDR which = 16
Both A and B include the full IP address range of both networks, though B does so with less over-summarizing. Is there any rule in supernetting / route summarization that would make B (10.8.0.0/19) ineligible for some reason? Because if there is, then A would be a better answer.
The Answer is A the only thing you need to know is what they have in common and that would be only the first 2 octets so CIDR notation tells you 16 uses the full range of the first 2 Octets.
There are not a IP address,you can not only consider the 10.8.31.0(this is not a IP address!!,is represent a subnet)
10.8.31.0/21=10.8.31.0--10.8.38.255
10.8.0.0/19=10.8.0.0--10.8.31.255,this answer B doesn't cover 10.8.32.0--10.8.38.255
so only can choose 10.8.0.0/16=10.8.0.0--10.8.255.255
From my calculations, when you go bit by bit and deduce the number of subnets for each, there are 16 subnets allowed for in 10.8.16.0/22 and 64 subnets allowed for in 10.8.31.0/21. Therefore, 16 networks would be the least common number, and A. 10.8.0.0 / 16 would be the answer. Three resources that helped me with IP addressing is Professor messer's 7 second subnetting on youtube, Master I.T.s lecture on subnetting, and then this for practicing problem after problem. http://www.subnettingquestions.com/
I don't understand why lots of people choose answer C? Please do subletting firstly and have a look, C and D don't overlap the two given networks, and B is the smallest(best) subletting here can contain both of them.
10.8.16.0/22 10.8.16.0 - 10.8.19.255
10.8.31.0/21 10.8.16.0 - 10.8.31.255
A. 10.8.0.0/16 10.8.0.0 - 10.8.255.255
B. 10.8.0.0/19 10.8.0.0 - 10.8.31.255
C. 10.8.0.0/20 10.8.0.0 - 10.8.15.255
D. 10.0.0.0/24 10.0.0.0 - 10.0.0.255
Just corrected as below, you can calculate the subnet by using any tools, it will be the same, but I would still choose B, A is too large. I also correct your anwser above.
10.8.16.0/22 10.8.16.0 - 10.8.19.255
10.8.31.0/21 10.8.24.0 - 10.8.31.255
So the range is at least from 10.8.16.0 - 10.8.31.255
you can see only A, B contains this range, but B is much smaller.
A. 10.8.0.0/16 10.8.0.0 - 10.8.255.255
B. 10.8.0.0/19 10.8.0.0 - 10.8.31.255
C. 10.8.0.0/20 10.8.0.0 - 10.8.15.255
D. 10.0.0.0/24 10.0.0.0 - 10.0.0.255
well said! very succinct too. the options provided for answers are specific, the Network ID is included, and overrides the number of bits. it could have been /20, if that was ALL they included in the choices, but they were specific.
The answer is A. /20 will give you the subnets of 10.8.0.0 - 10.8.14.255 // 10.8.15.0 - 10.8.29.255 and so on. /19 will give you 10.8.0.0 - 10.8.30.255 // 10.8.31.0 - 10.8.61.255 and so on.
For the networks provided in the question, you need to account for the subnets of 10.8.16.0 - 10.8.18.255 and 10.8.31.0-10.8.37.255. Hope this helps and if my math is wrong please let me know.
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TT
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