CIDR is the short form of Classless Inter-Domain Routing, which facilitates the allocation of IP addresses. In layman’s language, it is a set of internet protocol standards that uniquely identifies individual networks and devices. The IP ensures that specific information is sent to particular computers. CIDR comes in to offer more solutions in the allocation of IP addresses.

This has led to a significant increase in the number of IP available. Also, it has led to the widespread use of network translation hence extending the useful life of IPv4.

CIDR now replaces the old system based on classes A, B, and C. Each class allocated 32 bit IP address to identify the network gateway, the first 8 bits of class A, the first 16 bits of class B, and the first 24 bits of class C. the rest identifies the hosts on that network;

• Class A – Over 16 million host identifiers
• Class B – 65,535 host identifiers
• Class C – 254 host identifiers

However, a problem would arise if a company had more than 254 machines. This meant that it was hard to accommodate them in class C, and one had to do so in class B. An organization had to go ahead and subscribe to class B license even though they had far less than 65,535 hosts. For example, if a company only required 2500 hosts, they would be wasting at least 63035 hosts.

 Therefore choosing to hold a class B license wouldn’t be an ideal solution. In the end, this would decrease the availability of IPv4 available. CIDR has come to address the problem of space by providing a convenient way to specify network addresses in routers.

But if you are new to this field, you may be having numerous questions on how this invention works. Relax; we are going to use layman language to help you grasp everything about CIDR.

How Does CIDR Work?

CIDR is based on variable-length subnet masking (VLSM).  This facilitates in the definition of prefixes of arbitrary lengths, thereby making a more efficient method compared to the old system. The CIDR consists of two groups of numbers, also known as groups of bits. The network address in these groups is used to identify a network or a subnet. 

Well, it is important to note that the network address is written as a prefix. It looks like a normal IP address. For example, 192.255.255.255 is a network address.  The second part is a suffix, and it shows how many bits are in the entire address for example/12. So a normal CIDR IP address would be something like this 192.255.255.255/12. A /12 CIDR IP Block equates to 1,048,576 usable IPv4 addresses!

The network address is also part of the IP address. It tends to vary depending on the number of bits required. From our example, 12 bits are part of the address, while the rest 20 bits are the host addresses. Here are some more examples.

/29 IPv4 CIDR Block – 5 Usable IPv4 Addresses
/28 IPv4 CIDR Block – 13 Usable IPv4 Addresses
/24 IPv4 CIDR Block – 253 Usable IPv4 Addresses
/23 IPv4 CIDR Block – 509 Usable IPv4 Addresses
/22 IPv4 CIDR Block – 1021 Usable IPv4 Addresses
/21 IPv4 CIDR Block – 2045 Usable IPv4 Addresses

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Final Thoughts

As the adage goes, necessity is the mother of invention. CIDR has come a long way in improving the efficiency address allocation. The IP addresses were becoming exhausted in IPv4. This invention has extended the useful life of IPv4.