A subnetted Class B network has the following broadcast address : 220.127.116.11. Its subnet mask
(a) is necessarily 255.255.224.0
(b) is necessarily 255.255.240.0
(c) is necessarily 255.255.248.0
(d) could be any one of 255.255.224.0, 255.255.240.0, 255.255.248.0
My attempt :
For class B network broadcast address first two octet or 16 bits should be all are 1 for preserving class B network address (255.255.0.0) , now given address 18.104.22.168 is same as 144.16.010 11111.1111 1111 , clearly last 13 bits all are 1 contiguously , it shows that last 13 bits for hosts addresses , in third octet first 3 bits should be for subnet address . It should be preserve in broadcast address . So that 3 bits should all are 1 in subnet mask . therefore , resultant subnet mask will be 1111 1111. 1111 1111. 111 00000. 0000 0000 = 255.255.224.0
Hence , option (a) is true as per my calculation.
Can you explain it in a formal way, please?