What is CIDR?
CIDR, or Classless Inter-Domain Routing, is a method for more efficient allocation of IP addresses and improved routing on the Internet. It was developed to overcome the limitations of the original IP addressing system and to better utilize the limited resources of IPv4.
Why was CIDR introduced?
Before CIDR, IP addresses were divided into fixed classes: Class A, Class B, and Class C. This system led to inefficient usage, as many organizations were assigned more addresses than they actually needed.
How does Classless Inter-Domain Routing work?
Instead of using fixed classes, CIDR enables flexible division of IP address ranges through variable-length subnet masks (VLSM). This allows network administrators to divide networks into subnets of different sizes, leading to more efficient usage.
Example of CIDR notation
A typical CIDR address looks like this: 192.168.0.0/24. Here, the "/24" indicates that the first 24 bits form the network address and the remaining 8 bits are available for host addresses.
What advantages does CIDR offer for network administrators?
CIDR brings numerous advantages that make network administrators' work easier and increase network efficiency.
Key advantages of CIDR
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Efficient IP address usage:
Through flexible division, IP addresses are optimally used and waste is minimized. -
Improved routing efficiency:
Routers can process data packets faster as routing tables are smaller and more organized.
How does CIDR influence the transition to IPv6?
Although CIDR improves the efficient use of IPv4 addresses, the address space is not sufficient in the long term. IPv6 offers a significantly larger address space, and CIDR principles are also applied here to efficiently manage networks.
How are IP addresses assigned through CIDR?
The Internet Assigned Numbers Authority (IANA) manages global IP address spaces. It distributes large CIDR blocks to Regional Internet Registries (RIRs), which pass these on to Local Internet Registries (LIRs). From there, the addresses go to Internet Service Providers and finally to organizations.
What are supernets and how do they help?
Supernets are summaries of multiple smaller networks into a larger network block. Through CIDR, routers can manage these more efficiently, which reduces routing complexity and accelerates data transmission.
How do organizations use CIDR in practice?
Many organizations use CIDR to design their networks flexibly and securely. One example is Amazon Virtual Private Cloud, where companies can define their own IP address ranges and isolate their resources in the cloud. To stay with the Amazon example, it's worth looking at the AWS IP ranges - there you'll quickly see that Amazon uses CIDR blocks. Special services, for example EC2 instances, are available in specific CIDR blocks.
Example of EC2 CIDR from AWS
{
"ip_prefix": "35.180.0.0/16",
"region": "eu-west-3",
"service": "EC2",
"network_border_group": "eu-west-3"
},
It's easy to recognize the CIDR block "35.180.0.0/16", which is available for the "EC2" service in the "eu-west-3" region.
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