CIDR

CIDR
Name	CIDR
Inventors	Vinton Cerf, Paul Mockapetris, Van Jacobson
Introduced	1993
Standard	RFC 1519, RFC 4632
Related	Internet Protocol, IPv4, IPv6, Classful network, Routing table

CIDR

CIDR is a method for allocating and routing Internet Protocol addresses that replaced earlier classful addressing. It enables flexible address block sizes and reduced routing-table growth by permitting aggregation across multiple networks. CIDR has been central to Internet engineering, influencing work by standards bodies and operators across projects and institutions.

Overview

CIDR introduced a prefix-based system to represent address blocks and routes, permitting variable-length prefixes rather than fixed classes. It was developed in response to scaling problems identified by engineers in academic, industry, and standards communities including researchers at Stanford University, Lawrence Berkeley National Laboratory, and network operators at NSFNET and commercial carriers. Standards and operational guidance came through contributions to the IETF and documentation in RFCs, aligning with protocols such as BGP and routing implementations from vendors like Cisco Systems and Juniper Networks.

History and Rationale

The rationale for CIDR grew from work documenting address exhaustion and routing instability during the late 1980s and early 1990s. Influential events and reports from organizations such as ARPA, DARPA, NSF, and the Internet Society highlighted the need for a new allocation scheme. Engineers including those associated with Bell Labs, MIT, and network research groups proposed prefix aggregation to mitigate growth of routing tables observed on backbone routers operated by carriers like Sprint and UUNET. RFC 1519 formalized the technique amid widespread coordination with registries like IANA and regional registries such as ARIN, RIPE NCC, and APNIC.

Notation and Addressing

CIDR notation combines an IP address with a suffix indicating the prefix length, expressed as a slash followed by a decimal number. This notation harmonizes with addressing concepts used in the Internet Protocol family, mapping to subnet masks and prefix lengths familiar to implementers at companies like Microsoft, IBM, and Apple Inc.. Operators in data centers run by Amazon Web Services, Google Cloud, and Microsoft Azure commonly use CIDR blocks when assigning virtual networks. Registries and allocation policies from bodies such as LACNIC and AfriNIC govern assignment practices and prefix minimums.

Routing and Aggregation

CIDR enabled route aggregation (supernetting), reducing the number of routes propagated via interdomain protocols like BGP-4. Aggregation allowed service providers and backbone networks operated by Level 3 Communications and content networks like Akamai to advertise summarized routes, lowering forwarding table pressure on routers from vendors including Huawei and Arista Networks. The technique interacts with route selection processes defined by standards and operational guides used by engineers at RIPE NCC and network operators participating in MANRS.

Deployment and Impact

Deployment of CIDR across the global routing system occurred throughout the 1990s and into the 2000s, coordinated by registries, registrars, and large ISPs such as AT&T and Verizon Communications. Its impact included delayed exhaustion of IPv4 address space, changes in address allocation policy at IANA and regional registries, and modifications to routing software stacks in open-source projects like BIRD and Quagga (now FRRouting). The approach also influenced the design and adoption timeline of IPv6 by altering projections of remaining IPv4 address lifetime used by standards communities and operators.

Implementation and Examples

Network engineers implement CIDR in configuration snippets for routers and firewalls from vendors like Cisco Systems, Juniper Networks, and Palo Alto Networks; cloud orchestration from OpenStack and orchestration tools used by teams at Dropbox or Netflix rely on CIDR semantics. Common examples include aggregating contiguous /24 networks into a /22 for advertisement across a border with peers at Internet exchanges like LINX and AMS-IX. Examples used in educational materials from universities such as Carnegie Mellon University and University of California, Berkeley illustrate conversion between prefix lengths and subnet masks and show how routing policies use prefix lists and route maps to control aggregate announcements.

Limitations and Criticisms

Criticism of CIDR includes operational complexity for multihoming and traffic engineering, where providers and networks at organizations like Facebook and LinkedIn may advertise de-aggregated prefixes to influence inbound traffic. Some observers at research institutes including ICANN and IETF working groups argued that aggregation relies on stable upstream relationships and address allocation practices enforced by registries such as ARIN and RIPE NCC; violations lead to routing table growth. CIDR does not eliminate address scarcity concerns that motivated IPv6 deployment, nor does it remove the need for careful route filtering advocated by communities including NOGs and best-practice initiatives like MANRS.

Category:Internet standards