Computer Networking-How to Mathematical Calculation Network IP Mask, IP Subnet Mask, IP Addressing and Subnetting

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Network address is 192.168.1 and host address is 10.

In binary notation

Network address is 11000000.10101000.00000001 and host address is 00001010

Computer Networking How to Calculate Mask, Subnet Mask
Computer Networking How to Calculate Mask, Subnet Mask
IP Class Default Subnet Network bits Host bits Total hosts Valid hosts
A 255.0.0.0 First 8 bits Last 24 bits 16, 777, 216 16, 777, 214
B 255.255.0.0 First 16 bits Last 16 bits 65,536 65,534
C 255.255.255.0 First 24 bits Last 8 bits 256 254

 

Convert binary in decimal

To convert a binary in decimal we will follow above method in reverse mode. We will find the decimal value of on binary bit position and add them. For example convert 10101010 binary number in decimal.

Target binary number 10101010

Move direction From Left ===========================> to Right

Base position 27 26 25 24 23 22 21 20
Decimal value 128 64 32 16 8 4 2 1
Bit status 1 0 1 0 1 0 1 0
Decimal value in addition 128 0 32 0 8 0 2 0

Decimal value of 10101010 is 170 [ 128+0+32+0+8+0+2+0 ]

Default subnet mask

Class Subnet Mask Format
A 255.0.0.0 Network.Host.Host.Host
B 255.255.0.0 Network.Network.Host.Host
C 255.255.255.0 Network.Network.Network.Host


How many subnets does given subnet mask provide ?

To calculate the number of subnets provided by given subnet mask we use 2N , where N = number of bits borrowed from host bits to create subnets. For example in 192.168.1.0/27, N is 3. By looking at address we can determined that this address is belong to class C and class C has default subnet mask 255.255.255.0 [/24 in CIDR]. In given address we borrowed 27 – 24 = 3 host bits to create subnets. Now 23 = 8, so our answer is 8.

 

What are the valid subnets?

Calculating valid subnet is two steps process. First calculate total subnet by using formula 2N. In second step find the block size and count from zero in block until you reach the subnet mask value. For example calculate the valid subnets for 192.168.1.0/26.

Borrowed host bits are 2 [26-24].
Total subnets are 22 = 4.
Subnet mask would be 255.255.255.192.
Block size would be 256-192 = 64.
Start counting from zero at blocks of 64, so our valid subnets would be 0,64,128,192.

What are the total hosts?

Total hosts are the hosts available per subnet. To calculate total hosts use formula 2H = Total hosts. H is the number of host bits. For example in address 192.168.1.0/26 we have 32 – 26 [Total bits in IP address – Bits consumed by network address] = 6. Total hosts per subnet would be 26 = 64.
CIDR /25 has subnet mask 255.255.255.128 and 128 is 10000000 in binary. We used one host bit in network address.

Computer Networking How to Calculate Mask, Subnet Mask
Computer Networking How to Calculate Mask, Subnet Mask

 

N = 1 [Number of host bit used in network]
H = 7 [Remaining host bits]
Total subnets ( 2N ) :- 21 = 2
Block size (256 – subnet mask) :- 256 – 128 = 128
Valid subnets ( Count blocks from 0) :- 0,128
Total hosts (2H) :- 27 = 128
Valid hosts per subnet ( Total host – 2 ) :- 128 – 2 = 126

Subnets Subnet 1 Subnet 2
Network ID 0 128
First host 1 129
Last host 126 254
Broadcast ID 127 255

/26

CIDR /26 has subnet mask 255.255.255.192 and 192 is 11000000 in binary. We used two host bits in network address.

Computer Networking How to Calculate Mask, Subnet Mask
Computer Networking How to Calculate Mask, Subnet Mask

N = 2
H = 6
Total subnets ( 2N ) :- 22 = 4
Block size (256 – subnet mask) :- 256 – 192 = 64
Valid subnets ( Count blocks from 0) :- 0,64,128,192
Total hosts (2H) :- 26 = 64
Valid hosts per subnet ( Total host – 2 ) :- 64 – 2 = 62

Now Practice Ownself