CIDR stands for Classless Inter-Domain Routing. It is an IP address allocation method that improves data routing efficiency on the internet. Every machine, server, and end-to-end device that connects to the internet or an intranet has a unique number or address called an IP address, associated with it. Devices use these IP addresses to find and communicate with one another. Organizations use CIDR to allocate IP addresses flexibly and efficiently in their networks.
What are the different IP address formats?
An IP address has two parts:
- The network address is a series of numerical digits pointing to the network’s unique identifier.
- The host address is a series of numbers indicating the host or individual device identifier on the network.
Until the early 1990s, IP addresses were allocated using the classful addressing system. The total length of the address was fixed, and the number of bits allocated to the network and host portions was also fixed.
Classful addresses
An IPv4 address consists of 32 bits, Each string of number separated by the period consists of 8 bits, represented by 0 to 255 in numerical forms. Organizations could purchase three classes of IPv4 addresses.
Class A
A Class A IPv4 address has 8 network prefix bits. For example, consider 44.0.0.1 where 44 is the network address and 0.0.1 is the host address.
Class B
A Class B IPv4 address has 16 network prefix bits. For example, consider 128.16.0.2, where 128.16 is the network address and 0.2 is the host address.
Class C
A Class C IPv4 address has 24 network prefix bits. For instance, consider 192.168.1.100, where 192.168.1 is the network address and 100 is the host address.
Classless addresses
Classless or Classless Inter-Domain Routing (CIDR) addresses use variable length subnet masking (VLSM) to alter the ratio between the network and host address bits in an IP address. A subnet mask is a set of identifiers that returns the network address’s value from the IP address by turning the host address into zeroes.
A VLSM sequence allows network administrators to break down an IP address space into subnets of various sizes. Each subnet can have a flexible host count and a limited number of IP addresses. A CIDR IP address appends a suffix value stating the number of network address prefix bits to a normal IP address.
For example, 192.0.2.0/24 is an IPv4 CIDR address where the first 24 bits, or 192.0.2, is the network address.
What are the limitations of classful IP addressing that CIDR overcomes?
Before Classless Inter0Domain Routing (CIDR), IP addresses were classful and created inefficiencies, We discuss some of these shortcomings next.
Inflexible IP Addressing
In a classful addressing system, each class supported a fixed number of devices:
- Class A supported 16,777.214 hosts
- Class B supported 65,534 hosts
- Class C supported 254 hosts
The classful arrangement was inefficient when allocating IP addresses and led to a waste of IP address space.
For example, an organization with 300 devices couldn’t have used a Class C IP address, only permitting 254 devices. So the organization would’ve been forced to apply for a Class B IP address, which provided 65,354 unique host addresses. However, only 300 devices would’ve been connected, which would’ve left 65,234 unused IP ranges.
Limitations in network design
Classful IPs limit your ability to combine networks as required. For example, these IP addresses belong to different Class C networks in the classful architecture:
- 192.168.1.0
- 192.168.0.0
As a network administrator, you couldn’t have combined both networks because the Class C subnet mask was fixed as 255.255.255.0
What are the benefits of CIDR?
With Classes Inter-Domain Routing (CIDR), your organization has more flexibility in assigning IP addresses and routing data between devices.
Reduce IP address wastage
CIDR provides flexibility when you determine the network and host identifier assignments of an IP address. You can use CIDR to provision the required number of IP addresses for a particular network and reduce wastage. Besides, CIDR reduces routing table entries and simplifies data packet routing.
Transmit data quickly
CIDR allows routers to organize IP addresses to multiple subnets more efficiently. A subnet is a smaller network that exists within a network. For example, all devices connected to a router are on the same subnet and have the same IP address prefix.
With CIDR, your organization can create and consolidate multiple subnets. This allows data to reach the destination address without taking unnecessary paths/
Create a Virtual Private Cloud
A virtual private cloud (VPC) is a private digital space hosted within the cloud. It allows your organization to provision workloads in an isolated and secure environment. A VPC uses CIDR IP addresses when it transfers data packets between connected devices.
Create supernets flexibly
A supernet is a group of subnets with similar network prefixes. CIDR allows flexibility in creating supernets, which isn’t possible in conventional masking architecture. For example, your organization can combine IP addresses into a single network block using a notation like this:
- 192..168.1 / 23
- 192.168.0 / 23
This notation applies a subnet mask of 255.255.254.0 to the IP address, which returns the first 23 bits as the network address. The outer needs only one routing table entry to manage data packets between devices on the subnets.