Introduction to Networking

Subnetting In ClaasFul IP Addressing

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

  • The sizes of all sub-networks are the same.
  • Subnets of all subnetworks are the same.
  • All the sub-networks have an equal number of hosts.

Let’s understand with an example

Example-01

Suppose

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

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

To create two subnets (subnetworks) and to represent their subnet IDs, we require 1 borrowed bit from the 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: The number of subnets depends upon the number of borrowed bits. We can define this relation through the following formula.

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

If given, IP= 205.10.20.0. first, the IP Addresses of each of the 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