Computer Networking

Transport Layer

Subnetting In ClaasFul IP Addressing

As we already know that, The classful subnetting is also called Fixed length subnetting. The Fixed length subnetting contain  the following properties.

  • Sizes of all sub networks are same.
  • Subnets of all sub networks are same.
  • All the sub networks have equal number of hosts.

Let understand with example

Example-01

Suppose

  • We have a big single big network having IP Address 205.10.20.0.
  • We want subnetting. So, divide this network into 2 subnets.

As given IP belongs to Class C So, 24 bits are used for net ID and 8 bits are used for Host ID.

For creating two subnets (sub networks) and to represent their subnet IDs, we require 1 borrowed-bit from Host part.

So,

  • We borrowed one bit from the Host part. 
  • If borrowed bit = 0, then it will represent the first subnet.
  • If borrowed bit = 1, then it will represent the second subnet.

Note: After borrowing one bit, Host ID part remains with only 7 bits.

IP Address of the two subnets are

  • 10.20.00000000 = 205.10.20.0
  • 10.20.10000000 = 205.10.20.128

For First Subnet

  • IP Address of the subnet = 205.10.20.0
  • Total number of IP Addresses = 27= 128
  • Total number of hosts = 128 – 2 = 126
  • Range of IP Addresses = [205.10.20. 00000000 to 205.10.20. 01111111] = [205.10.20.0 to 205.10.20.127]
  • Direct Broadcast Address = 205.10.20.01111111 = 205.10.20.127
  • Limited Broadcast Address = 255.255.255.255

For Second Subnet

  • IP Address of the subnet = 205.10.20.128
  • Total number of IP Addresses = 27= 128
  • Total number of hosts = 128 – 2 = 126
  • Range of IP Addresses = [205.10.20.10000000 to 205.10.20.11111111] = [205.10.20.128 to 205.10.20.255]
  • Direct Broadcast Address = 205.10.20.11111111 = 200.1.2.255
  • Limited Broadcast Address = 255.255.255.255

Important Note: Number of subnets depends upon the number of borrowed bits. We can define this relation through the following formula.

Number of subnets = 2n. where “n” is number of borrowed bits. So, if number of borrowed bits are 2 then subnets will be 22 = 4.

If given IP= 205.10.20.0. then first IP Address of each of four subnets are 

  • 10.20.00000000 = 200.1.2.0
  • 10.20.01000000 = 200.1.2.64
  • 10.20.10000000 = 200.1.2.128
  • 10.20.11000000 = 200.1.2.192

 

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