SELFDOUBT IP

Question

SUPPOSE WE ARE GIVEN TWO BITS FOR SUBNET THEN WHICH ONE WILL BE THE THIRD SUBNET 10 OR 11

Answer 1

if you have 2 bit then there are 4 posiblities.

00--- subnet id

01--- first subnet

10---2nd subnet

11---3rd subnet

Example:

 

Comments

in one of the question of ace test series they asked the fourth octet of last ip address of the 3rd subnet for a class c network with subnet mask /26..

nd they marked the answer ad 190

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@abhisheskmehta when we do subnetting we assume  that for 2 bit(2^2-2=2 subnets) subnetting that 00 goes for 0th subnet and 01 (first subnet ) 10 (second subnet) and last one 11(DBA subnet) (we assume that network itself will not participate in subnetting).So how can u say that 11 is third subnet?If we assume that network can participate in subnetting then 2^2=4 subnets and 00(first subnet) 01(second subnet) 10(third subnet) 11(4th subnet).correct me if i am wrong.

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Its little bit ambiguous . But In gate question it is given first id is subnet id or total number of subnet = 2^n -2 .

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so as of now there will not be third subnet  for the given question because we follow the assumption that number of subnets=2^n-2 so only first and second subnets are possible.correct me if i am wrong.

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what if  the network has suppose 200 ip and we divide it as

1st subnet :- 100 ip

2nd subnet :- 50 ip

3rd subnet :- 50 ip.

Then we can divide the network into 3 subnets.

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@sabir this can be done using 2 bits with  the help of  variable length subnetting.two choices might be there, subnetting bit as 1 for first subnet and 00 for second subnet and 01 for third subnet or 0 for first subnet and 10 for second subnet and 11 for third subnet.thisquestion is different from the given question.

Answer 2

It depends upon the way in which you want to divide the network into subnets. 

Eg:- 200.254.254.0/26 is a class C network having 2 bits for subnet ID.

NOTE:- /26 is CIDR value which means number of 1s in the subnet mask.

In this the first 2 bits of last octet are for subnet i.e. 200.254.254.00000000.

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Now if you want to divide this network into two equal subnets then we need only the 1st bit of subnet bit and it would be divided as

200.254.254.0/25 to 200.254.254.127/25 -----> subnet 1--------> 200.254. 254 .00000000/25 to 200.254. 254 .01111111/25.

200.254.254.128/25 to 200.254.254.255/25 ---------> subnet 2------> 200.254. 254 .10000000/25 to 200.254. 254 .11111111/25.

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 if you want to divide subnet 1 into two equal subnets then we need  the 1st and 2nd bit of subnet bit and it would be divided as

200.254.254.0/26 to 200.254.254.63/26 -----> subnet 3--------> 200.254.254.00000000/26 to 200.254.254.00111111/26.

200.254.254.64/26 to 200.254.254.127/26 -----> subnet 4--------> 200.254.254.01000000/26 to 200.254.254.01111111/26.

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Similarly, if you want to divide subnet 2 into two equal subnets then we need  the 1st and 2nd bit of subnet bit and it would be divided as

200.254.254.128/26 to 200.254.254.191/26 -----> subnet 5--------> 200.254.0.10000000/26 to 200.254.254.10111111/26.

200.254.254.192/26 to 200.254.254.255/26 -----> subnet 6--------> 200.254.254.11000000/26 to 200.254.254.11111111/26.