CIDR
/SY-der/ · noun · Development · Origin: 1993
Definitions
Classless Inter-Domain Routing — a method of allocating IP addresses and routing that replaced the wasteful Class A/B/C system. CIDR notation (e.g., 10.0.0.0/24) uses a prefix length to specify the network portion of an address, enabling more efficient use of the IPv4 address space and reducing routing table sizes.
In plain English: A way of writing IP address ranges using a slash and number (like /24) that tells you how big the network is.
Example: The security group allows inbound traffic from 10.0.0.0/16, meaning any IP address starting with 10.0.
Origin Story
The slicing scheme that saved the internet from running out of routes
**Classless Inter-Domain Routing** was introduced in 1993 (RFC 1518, 1519) to replace the old Class A/B/C addressing system. The internet's routing tables were growing uncontrollably, and Class B addresses (65,534 hosts each) were being wasted by organizations that needed only a few hundred.
CIDR eliminated rigid address classes and introduced **variable-length subnet masking**. Instead of a Class C giving you exactly 254 addresses, CIDR notation like `192.168.1.0/24` lets you specify any power-of-two block size. A `/28` gives 14 addresses; a `/16` gives 65,534.
More importantly, CIDR enabled **route aggregation** -- ISPs could advertise a single route covering thousands of customer networks. Without CIDR, the global routing table would have exceeded router memory by the late 1990s, effectively crashing the internet.
Coined by: Vince Fuller, Tony Li, Jessica Yu, Kannan Varadhan
Context: RFC 1518/1519, 1993
Fun fact: The 'classful' system CIDR replaced was designed in 1981 when 4.3 billion addresses seemed infinite. Class A networks (16 million addresses each) were handed out to single organizations like MIT, HP, and the US Postal Service. Many still hold them.