Networking-IP

Question

If Direct Broadcast Address of subnet is 201.15.16.31. Which of the following will be subnet mask ?

1. 255.255.255.240

2. 255.255.255.192

3. 255.255.255.198

4. NOTA

Explain your answer

Comments

How? What is the reason

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Option b is given as 192 and not 240 in the 4^th octet.

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how did you find subnet mask @Pradeep Verma

Answer 1

There will be multiple answer for this..like 255.255.255.224, 255.255.255.240, 255.255.255.248 ans so on till /31

Answer 2

For a Direct Broadcast Address host bits will always be 1's.

We have DBA 201.15.16.31 so we can guess that There would a class C IP address 201.15.16.X and Definitely We have done subnetting there otherwise as default mask for class C is 255.255.255.0 so in that way last octet is for host and for DBA all host bits would be converted to 1 so in that way last octet becomes 255 but as it's 31 so certainly subnetting is done and subnet bits are borrowed from host portion.

Now if we borrowed 1 bit from host portion (we borrow from left) then DBA last octet will have at least last 7 bits 1 but in 31 it's not the case

now if we borrowed 2 bit from host portion then DBA last octet will have at least last 6 bits will be 1 but that's also not the case (31 =>00011111)

Now if we borrowed 3 bits from host portion then DBA last octet will have at least last 5 bits will be 1 and yes in our given DBA last 5 bits are 1 so subnet mask can be 255.255.255.224 (/27)

Now if we borrowed 4 bits from the host portion then DBA last octet will have at least last 4 bits will be 1 so that's also possible.

and so on.

Comments

Even 255.255.255.198 can also be Subnet mask as it can be viewed as /28 ?

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255.255.255.198 is /26 not /28.

No it can't. as if it is one possible subnet mask then DBA would for sure contain last 6 digits 1. which is not a case here in given DBA.

Answer 3

DBA is represented as valid Network ID + all hosts bits will be 1's (i.e series of 1's)
I am representing last 2 octet in binary here to decide the network/subnet bits and hosts bits 

201.15.16.31 = 201.15.00010000.00011111

So definitely if it is a DBA then at most last 5 bits can be host bits

So here we can have these possible answers 

Network/Subnet ID    Network/Subnet Mask    Hosts configured

201.15.16.0                 255.255.255.224              30 hosts
201.15.16.16               255.255.255.240              14 hosts          
201.15.16.24               255.255.255.248              6 hosts
201.15.16.28               255.255.255.252              2 hosts
201.15.16.30               255.255.255.254              0 hosts