Question and answer explaination

[daniel4579329]

can someone explain to me the question and how to get to the answer

thanks

[Martin L · Accepted solution]

they asking for 64 subnets (not hosts) and gave you /16 network; so,

1. first step is to figure out what number gives u 64 when 2 is raised to power of.  6, 2 raised to power of 6 gives us 64.

2. this means that u must borrow 6 bits from network part of /16, which is /22.

3. convert /22 to subnet decimal dot format, that is 255.255.252.0

The answer is 4th one from the top:  255.255.252.0

If you have to do this for hosts, you will count from the end (32- x) and must take 2 IPs away (2^x -2).

Regards, ML
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[daniel4579329]

hi
can you explain to me the proccess if they would ask me for 64 host in every subnet (would it be 255.255.255.192 ? )

[Richard Burts]

If they changed the question and asked for 64 hosts in each subnet you could use logic similar to what ML suggested about subnets. for 64 hosts what power of 2 produces 64 as the result? It is 6. So you would need 6 host bits, leaving you 2 network bits in the fourth octet. And the mask would indeed be 255.255.255.192.

[Richard Burts]

The questions is asking about how to do subnetting to achieve a particular result. There are 2 important items in the question. The first is that the address space is 172.16.0.0/16. So you will be subnetting in the third octet. The second important thing is that you want to get 64 subnets. The question asks what would be the subnet mask of devices in this network. The question provides 5 possible answers. 

There are 2 ways to find the right answer. You might use the process of elimination by identifying options that are not correct. Looking at the 5 possible answers the first option looks like an IP address and is not a valid subnet mask 172.16.64.0. The last option 255.255.64.0 looks like a subnet mask but is not a valid subnet mask (in a valid subnet mask all 1s must be contiguous and all 0s must be contiguous - and .64 does not meet this condition). The option for 255.255.255.0 would result in 256 subnets and so is not the correct answer. The option for 255.255.254.0 would result in 128 subnets and so is not the correct answer. The option for 255.255.252.0 would result in 64 subnets and is the correct answer.

The other approach is to logically find the correct value. If you are subnetting in the third octet and want to produce 64 subnets how big should each subnet be? If the third octet has 256 numbers and you divide them into 64 groups, then how big is each group? It would be 4. So what subnet mask provides subnets that increment by 4 (0, 4, 8, 12, etc)? It is 255.255.252.0