IPv4 Blocks and CIDR Netting

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The Internet Engineering Task Force (IETF) in 1993 introduced CIDR or Classless Inter-Domain Routing or Classless Addressing in order to slow down the fast exhaustion of IPv4 address blocks and the expansion of internet routing tables on routers (IPv4 Blocks and CIDR Netting).

IPv6 Allows For Bigger Payloads Than IPv4

In networking basics, CIDR or Supernetting is nothing but a bit-wise representation of IP addresses. It allows to group (known as CIDR group) IP addresses into single routing table entries. CIDR is used at the ISP or IANA or RIR’s and works somewhat like subnetting. Subnetting is where an IP block or class is divided into smaller IP groups at an organizational level. However, CIDR and subnetting work on the same concept of making smaller groups of IPs in order to reduce wastage of IPs and use only the ones which are needed (IPv4 Blocks and CIDR Netting).


Read More on IPv4 Subnetting and VLAN Task

IPv4 Blocks And CIDR Netting | Subnet Mask | Classful Addressing


Why was CIDR introduced?

Before the introduction of CIDR, A to D IP address classes were used to assign IP addresses and was known as Classful Addressing. The problem with classful addressing was that it was inefficient in allocating address space, by classifying them into IP range and in turn replacing the Classful addressing. You can see in the table below how Classful addressing was done and the CIDR notation making it Classless addressing:


IP Class

Size of Bit

IP Range

Subnet Mask (dot-decimal notation)

No. of Networks

No. of Hosts per Network

CIDR Notation (After CIDR Introduction)



1 – 126

128 (27)

16,777,216 (224)




128 – 191

16,384 (214)

65,536 (216)




192 – 223

2,097,152 (221)

256 (28)
















IPv4 Subnetting And VLAN Task
IPv4 CIDR identification: 

For easy identification of IP addresses and their path, the following method is used: dotted-decimal address (IPv4 address as a prefix), then a slash, then any integer from 0 to 32 (since IPv4 has 32-bit address length). 


An ideal CIDR example looks like this:

In CIDR, the network address holds importance since it helps in recognizing the network or subnet (an IP network subdivision). In this, the least bit group is the host identifier (the host or device where the data packets are received). Since CIDR is a flexible routing system it has been popularly used by ISPs to allocate IP blocks or IP addresses depending on their requirements. CIDR notation is the standard used now.

Now, IANA (Internet Assigned Numbers Authority) is the one who allocates CIDR block to RIR’s (Regional Internet Registry) who then allocates them to ISP’s and then to corporations and businesses (IPv4 Blocks and CIDR Netting). 

Apart from CIDR, to control the number of IPs being used within an organization and control the exhaustion of IPv4’s, NAT (Network Address Translation) was introduced. However, when IPv6 was introduced to counter the run out of IPv4s, there was another problem, the communication between an IPv4 device with an IPv6 device. To counter this problem, Dual-Stack is now being used by many, including Alpha InfoLab Inc. (Official IPv4 Brokers).


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Dual-Stack establishes communication, from IPv4 device to IPv6 device, IPv6 device to IPv4 device, IPv6 device to IPv6 device and IPv4 to IPv4. This is the most reliable communication system until IPv6 completely overtakes IPv4. 

Read More on IPv4 and IPv6 Dual-Stack

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