What Is CIDR?

Classless inter-domain routing (CIDR) is a method of internet protocol (IP) addressing by using subnetting and subnet masks to segregate classful IP addresses. Before 1993, a set of IP classes determined the routing protocol, leading to the fast consumption of IPv4 addresses because of the inefficiency of the routing protocol. The three main classes are Class A, Class B, and Class C. Each IP address has a specific number of bits in the network prefix and host number that says how many hosts are in that specific class. 

After 1993, the introduction of CIDR bypassed the need for classes since subnet masks were able to split up the three main classes. This process, called variable-length subnet masking (VLSM), lets IP addresses divide into subnetworks (subnets) all with the same network identifier, but each subnet contains a different number of hosts able to connect to that IP address. This increases network security. CIDR allows for supernetting, which is a routing method that brings separate subnets into one large network. 

Read more: Information Technology (IT) Terms: A to Z Glossary

Supernetting

One of the main advantages of CIDR is the ability to create supernets. CIDR uses subnetting to divide up a single network, but it can also create a supernet of those subnets. This process could summarize an organization’s routing with one IP address, called route summarization. To do this, you strategically borrow bits from the network prefix in the IP address to create the summarization.

CIDR notation

With the creation of CIDR also came CIDR notation, which is a simpler way to write an IP address that also contains how many bits are in its subnet mask. Here is an example of how CIDR notation writes an IP address with its respective subnet mask:

192.168.127.0/20

CIDR notation is sometimes called slash notation because of the use of a slash (e.g., /20) to indicate where the network prefix ends and the host numbers start. CIDR notation can drop trailing zeroes at the end of an address. So we can further simplify:

192.168.127/20

This quickly tells you that the network prefix is 192.168.127.0 and there are 20 bits in the network prefix. Since we are working with a 32-bit IPv4 address, that leaves 12 bits left to indicate how many hosts can connect to the IP address. This IP address has a 20-bit subnet mask of 255.255.240.0 and has 4,096 hosts and 4,094 usable hosts. You can calculate the number of hosts by hand using binary notation or by using a subnet calculator [1]. 

This example has been using an IPv4 address, but CIDR notation is also used for IPv6 addresses that can contain up to 128 bits.

CIDR address blocks

One advantage of CIDR notation is the easy identification of CIDR address blocks and how many hosts are in an IP address. The slash notation is the CIDR block. Staying with our example, the “/20” is a CIDR block that will always contain 4,096 hosts, no matter the IP address network prefix [2]. This allows for the quick reference of usable IP addresses, the amount of bits needed to create those hosts, and the subnet mask necessary for the division. 

Related terms

• Network administrator

• Subnet masking

• DHCP

• Router

• TCP/IP

• DDoS attack

Learn more about IT

Ready to learn more about IT? Learn about systems administration and IT infrastructure with a Google IT Support Professional Certificate on Coursera. This program is designed for beginners with no prior experience to develop the skills you need for a career in IT. The topics covered include technical support fundamentals, operating systems, IT security, and more.