CIDR(Classless inter Domain Routing)

What Is CIDR?

CIDR is a new addressing scheme for the Internet which allows for more efficient allocation of IP addresses than the old Class A, B, and C address scheme.

Why Do We Need CIDR?

There is a maximum number of networks and hosts with a new network being connected to the Internet every 30 minutes the Internet was faced with two critical problems: 

• Running out of IP addresses 
• Running out of capacity in the global routing tables 

Running Out of IP Addresses

There is a maximum number of networks and hosts that can be assigned unique addresses using the Internet's 32-bit long addresses. Traditionally, the Internet assigned "classes" of addresses: Class A, Class B and Class C were the most common. Each address had two parts: one part to identify a unique network and the second part to identify a unique host in that network. Another way the old Class A, B, and C addresses were identified was by looking at the first 8 bits of the address and converting it to its decimal equivalent. 

Address Class	# Network Bits	# Hosts Bits	Decimal Address Range
Class A	8 bits	24 bits	1-126
Class B	16 bits	16 bits	128-191
Class C	24 bits	8 bits	192-223

Using the old Class A, B, and C addressing scheme the Internet could support the following: 

• 126 Class A networks that could include up to 16,777,214 hosts each 
• Plus 65,000 Class B networks that could include up to 65,534 hosts each 
• Plus over 2 million Class C networks that could include up to 254 hosts each 

(Some addresses are reserved for broadcast messages, etc.). Because Internet addresses were generally only assigned in these three sizes, there was a lot of wasted addresses. For example, if I needed 100 addresses I would be assigned the smallest address (Class C), but that still meant 154 unused addresses. The overall result was that while the Internet was running out of unassigned addresses, only 3% of the assigned addresses were actually being used. CIDR was developed to be a much more efficient method of assigning addresses. 

Global Routing Tables At Capacity

A related problem was the sheer size of the Internet global routing tables. As the number of networks on the Internet increased, so did the number of routes. A few years back it was forecasted that the global backbone Internet routers were fast approaching their limit on the number of routes they could support. 

Even using the latest router technology, the maximum theoretical routing table size is approximately 60,000 routing table entries. If nothing was done the global routing tables would have reached capacity by mid-1994 and all Internet growth would be halted.

How Were These Problems Solved?

Two solutions were developed and adopted by the global Internet community: 

• Restructuring IP address assignments to increase efficiency 
• Hierarchical routing aggregation to minimize route table entries