ip address

Question

A network with a subnet mask_____,_____ subnets and in each subnet there are 32766 valid host address?

A)255.255.128.0,2

B)255.255.128.0,512

C)255.255.0.0,1

D)A or B

I am getting B but ans is A how plz explain?

Answer 1

A)
255.255.128.0
255.255.10000000.00000000
 last 15 bits will tell the host bits so  no os hosts possible with this 2¹⁵ -2=32766
underlined 1 means we have 2 subnets 
B) option cannot be true 
                  

                       cannot be true because we cant have 512 subnets with one bits

C) option is also not correct because we can form two subnets but this option says we can form only 1 subnet
A) option answer

Answer 2

32766 is 2¹⁵  so 15 bits for the hosts

assuming the network as  B (you can have class A also but options all are in Class B )

so network id will have 16 bits so left with 16 more but 15 are for hosts so 1 left for subnets so number of subnets will be 2 , so option A

Answer 3

Subnet Mask = 1's (Network ID + Subnet ID)  + 0's (Host ID)

A) 255.255.128.0, 2 :

11111111.11111111.10000000.00000000

having 2 subnets: means last one 1's is subnet ID

No of hosts in each subnet : 2^{(no of zeros)} -2 = 2¹⁵ -2 = 32766    (class B network)

B) 255.255.128.0, 512: 

11111111.11111111.10000000.00000000

having 512 subnets: means last nine 1's is subnet ID

No of hosts in each subnet : 2^{(no of zeros)} -2 = 2¹⁵ -2 = 32766   (class A network)

C) 255.255.0.0, 1 :

11111111.11111111.00000000.00000000

having 1 subnets: means network is not subnetted.

No of hosts : 2^{(no of zeros)} -2 = 2¹⁶ -2 = 65534

Answer is D, A or B both satisfying.