MadeEasy Subjectwise test

Question

Consider a subnet of class B network which has 160.24.223.255 as broadcast address. Consider the following subnet masks:

M1: 255.255.208.0
M2: 255.255.224.0
M3: 255.255.240.0
Which of the above subnet masks can be the subnet mask of given subnet?

How to solve such questions? What is the logic/process?

Answer 1

in a class B network 16 bits are for host id part and 16 bit for network id part

so network id of given class B network is 160.24.0.0

broadcast address of one of the subnet of this network is 160.24.223.255

it is directed broadcast address in fact because it has all ones in host id part of the subnet

expanding only hid part to analyze

160.24.11011111.11111111

so if it it DBA then it must have all 1's in HID part of subnet and since it has 13 Least significant bits as  1's we may confirm that hid part has max 13 bits means it may be less than that also.

 

case 1) : assuming no of bits in hid part = 13 then 

16 bits are NID part and 3 bits are for subnet id part hence,

starting 19 bits will be subnet mask (all ones in NID bits and subnet id bits)

so

subnet mask maybe

11111111.11111111.11100000.00000000   (255.255.224.0)

 

case 2) : assuming no of bits in hid part = 12 then 

16 bits are NID part and 4 bits are for subnet id part hence,

starting 20 bits will be subnet mask (all ones in NID bits and subnet id bits)

so

subnet mask maybe

11111111.11111111.11110000.00000000 (255.255.240.0)

similarly we may assume no of bits in hid part anything less than 13

and hence in subnet mask number of 1's will be 19 or more than that

so subnet mask will be anyone in

(255.255.224.0) or (255.255.240.0) or (255.255.248.0) and so on.

 

so here M2 and M3 are correct.