What Is CIDR Notation?

Published: December 15, 2016


Recently, I was involved in mitigating malicious scripted activity against a site that was found to be coming from a range of IP addresses. whois is a useful tool when dealing with this type of an issue. It can provide a network range for a given IP address.

$ whois

NetRange: -
NetName:        NET3-INC
NetHandle:      NET-104-232-32-0-1
Parent:         NET104 (NET-104-0-0-0-0)
NetType:        Direct Allocation
OriginAS:       AS36352, AS62584, AS55286
Organization:   Net3 Inc. (NETIN-11)
RegDate:        2014-10-27
Updated:        2014-10-27
Ref:            https://whois.arin.net/rest/net/NET-104-232-32-0-1

OrgName:        Net3 Inc.
OrgId:          NETIN-11
Address:        8195 Sheridan Drive
City:           Buffalo
StateProv:      NY
PostalCode:     14221
Country:        US
RegDate:        2013-07-10
Updated:        2015-08-14
Ref:            https://whois.arin.net/rest/org/NETIN-11

OrgTechHandle: NOC13226-ARIN
OrgTechName:   Network Operations Center
OrgTechPhone:  +1-289-408-9989
OrgTechEmail:  [email protected]
OrgTechRef:    https://whois.arin.net/rest/poc/NOC13226-ARIN

OrgAbuseHandle: NOC13226-ARIN
OrgAbuseName:   Network Operations Center
OrgAbusePhone:  +1-289-408-9989
OrgAbuseEmail:  [email protected]
OrgAbuseRef:    https://whois.arin.net/rest/poc/NOC13226-ARIN

OrgNOCHandle: NOC13226-ARIN
OrgNOCName:   Network Operations Center
OrgNOCPhone:  +1-289-408-9989
OrgNOCEmail:  [email protected]
OrgNOCRef:    https://whois.arin.net/rest/poc/NOC13226-ARIN

I provided the range of IP addresses ( - to the hosting company to block at the firewall. However, in their correspondence, they began referring to the IP address range in a way I wasn’t familiar with. It looked like this:

Curious as always, I did a little investigation and found out that this way of referring to networks is called CIDR notation. I became interested and decided to learn a little more about CIDR notation…what is it used for and why? Here, I’ll share my learnings for anyone else who is curious.

What Is CIDR Notation?

NOTE: We will only be talking about IPv4 in this post.

ALSO: This post doesn't go into detail on how CIDR notation is calculated. I recommend this post if that's what you're after.

In order to answer this question, it’s helpful to have a basic understanding of IP addresses and IP address classes.

The total range of possible IPv4 addresses spans from to This range is divided up into a number of classes.

Class A: -

For these IP addresses the first “octet” (set of numbers before the dot) is intended to refer to the network. The last 3 octets refer to the host. Class A IP addresses allow for only a few networks, but allow each network to have a massive amount of hosts.

Class B: to

In class of IP addresses the first two octets refers to the network and the last 2 octets refer to the host.

Class C: to

In this class of IP addresses the first 3 octets refer to the network and the last octet refers to the host. Each network can only have 255 hosts on this type of network.

IP addresses above are split into Class C and D, however those classes are used for multi-casting protocols and experimental use.

What Does This Have To Do With CIDR Notation?

To understand why CIDR notation exists you need to understand the problems that the class based IP routing system introduce. Namely it doesn’t adapt to the needs to actual real word networks. Class C networks can have 254 hosts, Class B networks, 65534 and Class A networks 16,777,214. Those are the only options. There are many networks that don’t fit into these moulds leading to many wasted IP addresses. In order so solve this subnetting was introduced as a technique for more control over network division.

I’m Still Not Following

CIDR notation is basically a way of referring to a subnet. For example, let’s take a look at the IP address range in question again: -

These are Class A addresses. In this case 104 should be the network and all the other numbers should refer to the host. However, there aren’t that many networks that have 16,777,214 hosts, so 104 was sub-divided into a number of smaller networks of which - is one. CIDR notation is a way to reference these subnets.

This is done by providing an IP address separated by a slash, and then (this next part’s tricky) the number of bits that reference the network portion of the address when the IP address is written in binary. As mentioned, I’m not going to go into detail on how that is actually calculated, just explain at a high level the purpose of this notation.

Why Should I Care About CIDR Notation?

If you work in business of keeping applications available online it is good to have some understanding of basic networking concepts. For example, your site may come under attack by a malicious network and you may work with a network engineer to resolve the issue.

Further, CIDR notation may enter your world beyond the firewall such as with mod_authz_host for Apache which lets you deny (or allow) networks using CIDR notation.


Here are some tools and resources that I found useful learning about CIDR notation.


I certainly don’t claim to be a networking guru, so it’s not unlikely that I got some things wrong above. Corrections and comments are certainly appreciated! Feel free to drop a note comments below, or, as always, you can reach me on Twitter as well.

Max Chadwick Hi, I'm Max!

I'm a software developer who mainly works in PHP, but loves dabbling in other languages like Go and Ruby. Technical topics that interest me are monitoring, security and performance. I'm also a stickler for good documentation and clear technical writing.

During the day I lead a team of developers and solve challenging technical problems at Something Digital where I mainly work with the Magento platform. I've also spoken at a number of events.

In my spare time I blog about tech, work on open source and participate in bug bounty programs.

If you'd like to get in contact, you can find me on Twitter and LinkedIn.