ME-CN-Practice Book-Q45

Question

My doubt---Whether # of subnet is 2^{bits for Subnet} or 2^{bits for Subnet} -2 ??

Answer 1

Given address has no '/' notation so we follow classful addressing..Under this scheme the given address falls under class B address.So we know default mask for class B =  255.255.0.0

Now hence no of network bits = 16 = No of host bits

Now if we take 7 bits from host for subnetting purpose and we have hence 9 bits remaining for host within a subnet..

So no of subnetworks  =  2^{no of subnet bits} - 2

                                 = 128 - 2 

                                 = 126

The reason being if all subnet bits are 0's they correspond to network id and if all 1's this corresponds to the direct broadcast address of the network..Hence both of these cases are excluded , hence we subtract 2 for getting no of subnets.

Now  no of hosts        =  2^{no of host bits} - 2

                                =   512 - 2

                                =   510

The reason being the first address corresponds to subnet id and last address corresponds to direct broadcast address of subnet hence both are excluded.

Hence C) is the correct answer.

                        

Comments

doubt 1---

is All subnet bit 0 is network address or all hostid bit 0 is network address??

doubt 2--

is all subnet bit 1 is broadcast add or all host id 1 is broadcast address

I mean network add and broadcast address depends on hostid,not only subnetid...right??

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Answer to ur 1st doubt :

All 0's in subnet bits will correspond to network id so it is not included in subnet id ..And the case if all host bits are 0's in a given subnet then it is referring to the subnet id of the subnetwork concerned just like network id for an entire network.

 

Answer to ur 2nd doubt :

If all subnet bits are set to 1 , then it becomes part of direct broadcast address of the entire network , hence this is not included in subnet id as well..And if all host bits of a given subnetwork is set to 1's , then it refers to direct broadcast address of that particular subnetwork not the entire network..

Hope this clears ur doubt @KISHALAY DAS

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Then answer should be c

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Ya u r correct..Editing ..Just slip of eye..:)

Answer 2

http://www.cisco.com/c/en/us/support/docs/ip/routing-information-protocol-rip/13790-8.html

we should not decrease the no of subnets by 2

according to me the options are wroong