Classless InterDomain Routing (CIDR)

Learned in ECE358.

https://aws.amazon.com/what-is/cidr/

Motivation: The problem with classful ip addressing: 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 spaces.

For example, an organization with 300 devices couldn’t have used a Class C IP address, which only permitted 254 devices. So, the organization would’ve been forced to apply for a Class B IP address, which provided 65,534 unique host addresses. However, only 300 devices would’ve been connected, which would’ve left 65,234 unused IP address spaces.

CIDR allows network routers to route data packets to the respective device based on the indicated subnet. Instead of classifying the IP address based on classes, routers retrieve the network and host address as specified by the CIDR suffix.

CIDR blocks A CIDR block is a collection of IP addresses that share the same network prefix and number of bits. A large block consists of more IP addresses and a small suffix.

I'm confusd, isn't this just 2-layer subnet?

Like a CIDR block is just a higher level subnet that contains multiple subnets. Yas.

• Whereas Class A,B,C are also subnetting, but fixed subnets of either /8, /16, or /24
• You can’t have a subnet of /23

https://stackoverflow.com/questions/43500344/cidr-vs-classful-subnetting Basically, ignore subnetting. It’s about network classes, and aborting those.

Like companies had to buy either from a network class A, network class B, or network class C. However, that can be super expensive / inefficient use. Let’s say you need 300 devices. You had to go and buy class C.

Okay, i’m still a little confused.