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Subnetting Guide

Subnetting is a process of breaking down a large IP Network into smaller sub-networks for efficient utilization. This article is about Subnetting tutorial.

What is Subnetting ?

Subnetting is the process of breaking down a large IP Network into smaller sub-networks for efficient utilization of limited resource of IP address pool. As defined in IPv4 standard, an IP address is a 32 bit address, sub-divided into four pairs of octets, which collectively serves as a unique address for any

Why Subnetting is required ?

In the early days of networking, the networks used to be smaller and networking devices used to be costly and larger in size. It was hard to predict the future of networking at that time. When Classful addressing was introduced, only three classes namely Class A, B & C were existent.

But it didn’t take much time to realize the inefficiencies of the existing system and an urgent need for an efficient solution to compensate those inefficiencies of classful addressing system. This gave rise to Subnetting.

In what ways, can subnetting be represented ?

There are two ways by which Subnetting can be represented.

  • Subnet Mask
  • CIDR (Classless Inter Domain Routing)

Subnet Mask : As IP addresses are 32 bit addresses paired in 4 octets, subnet mask is also a 32 bit mask, that has ‘1’ bit corresponding to a network bit as well as subnet bit and ‘0’ bit corresponding to a host bit of the IP address.

CIDR : In this method, the number of 1’s in Subnet mask is simply counted and the final count is appended at the end of  the subnetted IP address.

An Example

For instance, if a Class A IP address is then its first 8 bits represents Network bits and next 24 bits represent Host bits. Now, to make this addressing scheme more efficient for smaller networks, we can designate the 16 bits after Network bits as Subnet bits. This way, only 8 bits are left as Host bits. Now, this scenario can be represented with a subnet mask of

In CIDR format, referring to the above example, as there are 24 bits in Subnet Mask, the subnetted IP address will be represented as

Automated Subnetting

Although subnet calculation can be done manually but there are automated tools, such as Subnet Calculator, which can be put to use, whenever you want to quickly automate the subnetting process.

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