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

1. $255.255.0.0$

2. $255.255.64.0$

3. $255.255.128.0$

4. $255.255.252.0$

edited | 2.9k views
+3
$Remark:$
To divide a big network into many sub-networks, subnet bits will be borrowed from the host bits.

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

edited
+5

but using 6 bits we can make only 62 subnets (26 -2) ...?

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@skrahul   there is no need to subtract 2 here "Subnetid and DBA" will be counted in it nothing mentioned in question
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But we can not give these 2 addresses to any department..!!
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@ Arjun sir , sir please help me here . "wishes to form " is it giving any special impact to this question . and why didnt we subtracted 2 subnets which will be  DBA and Net Id ?  is it saying about zero subnet .?

thank you .

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@bharti, 255.255.254.0 is not in option so only choice is 255.255.252.0
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yup....u r ri8  @reena
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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  ....

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.

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

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Wishes to form means zero subnet and DBA subnet

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.

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