How to calculate Subnets

           CALCULATING NETWORK ID ON FINGERTIPS

This is a very simple technique which you will never find in any other sites or books, While attempting any exams we are asked to Calculate the Network id of the given IP. It's not a recommended way to use pen, book and calculations to find out the Ans. SO, Here come's a simple Formula to calculate that within seconds.

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*As the General subnets we can do easily, But this process is very useful in calculating subnets such as  192.168.2.181/30     * * ==> The network ID of the subnet is 192.168.2.180 we can calculate that in less than 7 seconds with this method                  *
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Follow the steps below:

1) Identify the CIDR value (number of network bits).
2) Identify the near by Round of value for that CIDR value.
    For example if the CIDR value is "/19" , then "/16" will be the near by Round off value.

    /8    /16   /24  /32 --> Round off Values Of CIDR Bits.
     A round off value is a standard number of network bits, which was not subnetted ( NO CIDR, No VLSM )

3) Subtract the extra bits subnetted than the Round off Value.
     In case of "/19", Extra bits Occupied by subnetting are "3". As "/16" is standard value. 1     16+3=19.
    ==> Say it n, So n=3.

4) Now calculate 2^n (2 to the power of n)
    ==>  2^3=8.

5) Last octet of IP address / n (Division)
    ==>  If consider 10.1.1.2 ==> 2/8 ==> 0.666 (We should never consider decimals.)

6) Multiply the n with the value we got above i.e., "0". 8*0 = 0.

7) IP: 10.1.1.2/19
    ==> Hence, Network ID: 10.0.0.0


Why we have taken 10 and the remaining zeros??
As the subnet mask is 255.0.0.0 ==> the first octet will never change so remaining bits are filled with zero, i.e., the answer we got.!!

7)Multiply n with the result we obtained.

Let's look at some examples:

Example 1:
IP: 172.16.1.3/20

1)CIDR value = /20.
2)Network ID: Round off value = 16
3)n=20-16 ==> 4.
4)2^4=16
5)3/16 = 0.
6)4*0 = 0  ==> Network ID
Hence the network ID will be: 172.16.0.0!

Example 2:
IP: 192.168.1.65/26

1)CIDR value = /26
2)near by round off value: /24
3)n=26-24 = 2.
4)2^2 = 4.
5)65/2 = 32.5 (Don't consider decimals)
6)32*4 = 64!! ==>Network ID.
Hence The network ID will be 192.168.1.64!

Example 3:
IP: 192.168.147.94/29

1) CIDR value =29.
2) Near by Round off value = 32 
Note: Near by value is not 24, as /24 is 5 bits far where /32 is only 3 bits far.
3) n = 32-29  ==> 3.
4) n= 2^3 ==> 8
5) 94/8 ==> 11.75    (as usual should not consider decimals)
5) Multiply n with the above value.
6) 8*11 = 88 ==> Network ID.
Hence the network ID will be 192.168.147.88!

This method is still under R&D , if you have any challenges please mail me.