CIDR Calculator

by Lochotin

IP Address

CIDR Mask Bits

Subnet Mask

Wildcard Mask

Maximum Subnets

Maximum Addresses

IPv4 Network Address

CIDR Route Address

Subnet Address Range


Classless Inter-Domain Routing

So what is CIDR?

I don't think it's necessary to go into a lot of detail here about the history and technical implementation of CIDR (Classless Inter-Domain Routing) as you are, without doubt, already familiar with the concept and its reasons for being implemented.

Briefly, in the early to mid 1990s, the spread of Internet usage was such that the routing equipment used at the time was quickly running into memory and performance problems due to the vast amount of data maintained in its routing tables.

At the same time the limited amount of address space available was quickly being used up - allocated to companies based on "classful" sub-netting standards.

Clearly a solution was required

Enter CIDR Stage Left

CIDR employs VLSM (Variable Length Subnet Masking) and address aggregation to enable routers to reduce the amount of memory used by routing tables. At the same time it helps with the allocation of public IP addresses by superseding the old Classful subnetting practices.

But was CIDR enough?

Well, in a word, no.

You see, the existing IPv4 address space was so fragmented that, when the time came to reallocate old network addresses, it was decided to try to do so in a fashion that would enable geographical addressing. This would help to ease the task of routing even more.

So existing allocated networks were replaced with smaller networks which better fulfilled the users requirements.

IPv4 Address Space Reallocation

To understand why this was necessary, it may be interesting to take a quick look at a real world example.

The Establishment I have in mind is one that I worked for for a number of years in the early to late 90s.

Originally, this establishment was allocated a Class A network and this was used, as was common practise at the time, to provide a public routable IP Address to every device on their worldwide network.

Given that a Class A network provides more than 16 million addresses, this was obviously slightly overkill and an enormous waste of the valuable IPv4 address space.

During the reallocation, this establishment was issued with a contiguous series of Class C networks providing ample space for all public facing devices.


With NAT (Network Address Translation) in popular use to take care of internal devices, there is no limit to how many networks and devices they can use in their private domain.

Autonomous Systems

Another technology employed at the time was the use of Border Gateway Protocols enabling large network addresses, to be routed in a single shot. This is accomplished by assigning each entity's network address with an AS (Autonomous System) number

So you see that all our Subnet Addressing woes were not solved by CIDR alone, but using CIDR as one element of the current solution.

Then came IPv6 - but that's a story for another day.

About the CIDR Calculator

The calculator on this site is intended as a tool to help Network professionals and students alike in their task of calculating CIDR subnets.

This is a modern, professional subnet calculator.

Coming Soon - Fun for All

Why not take some time off from your busy day and take a look at a bit of networking history?

People often talk about 'supernet' calculators, but have you ever seen a historical example based on the early idea of classful supernets?

Believe me, you're going to love it. But be warned, it may drive you crazy!

When it's ready, you'll be able to find it at Supernet Calculator