UGC NET CSE | June 2019 | Part 2 | Question: 84

Question

You need $500$ subnets, each with about $100$ usable host address per subnet. What network mask will you assign using a class B network address?

• A. $255.255.255.252$
• B. $255.255.255.128$
• C. $255.255.255.0$
• D. $255.255.254.0$

Answer 1

ans is option 2

class B default net mask 255.255.0.0  (11111111  11111111  00000000  00000000)

to have 500 subnets  9 more bits are required to be included in subnet mask  

so subnet mask will be 255.255.255.128  ( 11111111  11111111  11111111  10000000)

each subnet mask has 7 bits for host-part   which are sufficient for 100 hosts

Comments

have 500 subnets  9 more bits are required to be included in subnet mask  
 can u explain these line ,nd whats concept do you use  for number of sumnet  is 9..?

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To create subnets we need to extend the default network part  in other words we need to capture bits from host id part and by taking n bits we can have 2^n different subnets                                                                                   e.g 2 bits can give 2^2=4 different combinations 00 01 10 11 ( 4 differnet subnets with number 0 1 2 3)

Answer 2

In subnet mask network and subnet bits represent by one and host bits represent by zero. 

Answer 3

In class B network the first 16 bits will be the network id and the rest of the bits will be for the host ids.

In subnetting the host id is further borrowed.for 500 subnets to be possible at least 9 bits should be borrowed from the host id part.when 9 bits are borrowed from the host id atmost 510 subnets will be possible.

From the rest of the 7 bits 126 hosts will be possible for each subnet.

Therefore option (b) is correct

Answer 4

The answer will be b) as here bo.of subnets is $2^9$

No. of bits in network id = 8+8+8+1 =25 and the rest 7 bits are host if will will be $2^7$. so we can have upto 512 subnets and each subnet can have upto 128 hosts