GATE CSE 2005 | Question: 27

Question

An organization has a class $B$ network and wishes to form subnets for $64$ departments. The subnet mask would be:

• A. $255.255.0.0$

• B. $255.255.64.0$

• C. $255.255.128.0$

• D. $255.255.252.0$

Comments

$Remark:$
To divide a big network into many sub-networks, subnet bits will be borrowed from the host bits.

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Subnets for 64 departments means 64 subnets or subnets with 64 hosts(departments) in each subnet. Please clear!

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@roh 64 departments means 64 subnetworks and each subnetwork will have (32 – 8-8-6) i.e 2^10 IP addresses and (2^10-2) hosts. 

Departments and organizations terms are used to denote subnetworks. 

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Suppose we have a router A having address 11111111.11111111.00000000.00000000/16

Then subnets would be addressed as ,

       1. 11111111.11111111.00000000.00000000/16  

       2. 11111111.11111111.00000100.00000000/16

       3. 11111111.11111111.00001000.00000000/16

       4. 11111111.11111111.00001100.00000000/16​​​​​​​

                       ….

     63. 11111111.11111111.11111000.00000000/16​​​​​​​

     64. 11111111.11111111.11111100.00000000/16

 

 

  

Answer 1

D is correct answer.

To form subnet for $64$ departments we need $6$ continuous bit and the value of  11111100 = 252.

Organization has class $B$ network so subnet mask would be 255.255.252.0

Comments

In older days we couldn't use the subnet-zero and subnet-one so usable subnets were 2ⁿ-2.

but current IOS has the ability to use subnet-zero and subnet-one  ....

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Prior to Cisco IOS 12.0, by default, Cisco routers wouldn't allow you to configure an IP address on the all 0s network on an interface. However, you could configure this using the ip subnet-zero command in Global Configuration Mode.

Now, after IOS 12.0, the ip subnet-zero command is the default on routers. Note that this command not only allows the all 0s subnet, but it also permits the all 1s subnet. And that's why you no longer have to subtract 2 when using the network formula.

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https://www.cisco.com/c/en/us/support/docs/ip/dynamic-address-allocation-resolution/13711-40.html

http://www.faqs.org/rfcs/rfc1878.html

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Wishes to form means zero subnet and DBA subnet

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That is in practical case here theoritical will do

Answer 2

In class B network

16 bit network id and 16 bits are for host id part.

Here we want 64 subnets so we have to borrow 6 bits from host id part.

Subnet mask will be all 1 in network id and subnet id part.

255.255.252.0

Option d is correct

Comments

How to know that 64 are subnet not host

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Given in question

Answer 3

D is correct answer.

In class B the network bits are 16 and host bits are 16. In subnetting the bits are borrowed from Host portion and as given " wishes to form subnets"  directs that 2^6 means 6 continuous 1's. We don't need to subtract 2 here as we wish to form subnets, therefore instead of Net ID and DBA of network we will use them for Zero subnet and DBA Subnet.

Answer 4

subnets for 64 departments

means 

you need 6 bits for subnets

here by 6 bits we mean with 6 unique bits we can have 64 sub networks

classB

16 bits NID rest 16 bits host ID

first 6 bits of the third octet will be used

mask

11111111.11111111.11111100.00000000

255.255.252.0