Classless Inter-Domain Routing (CIDR, pronounced "cider") was introduced in 1993 and is the latest refinement to the way IP addresses are interpreted. Year 1993 ( MCMXCIII) was a Common year starting on Friday (link will display full 1993 Gregorian calendar) An Internet Protocol ( IP) address is a numerical identification ( Logical address) that is assigned to devices participating in a Computer network It replaced the previous generation of IP address syntax, classful networks. Classful network is a term that is used to describe the network architecture of the Internet until around 1993. Specifically, rather than allocating address blocks on eight-bit (i. e. , octet) boundaries forcing 8, 16, or 24-bit prefixes, it used the technique of variable-length subnet masking (VLSM) to allow allocation on arbitrary-length prefixes. CIDR encompasses:

• The VLSM technique of specifying arbitrary length prefix boundaries. A CIDR-compliant address is written with a suffix indicating the number of bits in the prefix length, such as 192. 168. 0. 0/16. This permits more efficient use of increasingly scarce IPv4 addresses. Internet Protocol version 4 ( IPv4) is the fourth revision in the development of the Internet Protocol (IP and it is the first version of the protocol to be widely
• The aggregation of multiple contiguous prefixes into supernets, and, wherever possible in the Internet, advertising aggregates, thus reducing the number of entries in the global routing table. Supernetting is synonymous with Classless Inter-Domain Routing (CIDR although CIDR is rather just the concept that is implemented when subnetting or Aggregation hides multiple levels of subnetting from the Internet routing table, and reverses the process of "subnetting a subnet" with VLSM.
• The administrative process of allocating address blocks to organizations based on their actual and short-term projected need, rather than the very large or very small blocks required by classful addressing schemes.

IPv6 utilizes the CIDR convention of indicating prefix length with a suffix, but the longer address field of IPv6 made it unnecessary to practice great economy in allocating the minimum amount of address space an organization could justify. Internet Protocol version 6 ( IPv6) is an Internet Layer protocol for packet -switched internetworks. The concept of class was never used in IPv6.

CIDR blocks

CIDR is principally a bitwise, prefix-based standard for the interpretation of IP addresses. It facilitates routing by allowing blocks of addresses to be grouped together into single routing table entries. Routing is the process of selecting paths in a network along which to send network traffic In Computer networking a routing table, or Routing Information Base (RIB, is an electronic table (file or database type object that is stored in a These groups, commonly called CIDR blocks, share an initial sequence of bits in the binary representation of their IP addresses. The binary numeral system, or base-2 number system, is a Numeral system that represents numeric values using two symbols usually 0 and 1. IPv4 CIDR blocks are identified using a syntax similar to that of IPv4 addresses: a four-part dotted-decimal address, followed by a slash, then a number from 0 to 32: A. B. C. D/N. The dotted decimal portion is interpreted, like an IPv4 address, as a 32-bit binary number that has been broken into four octets. The number following the slash is the prefix length, the number of shared initial bits, counting from the left-hand side of the address. When speaking in abstract terms, the dotted-decimal portion is sometimes omitted, thus a /20 is a CIDR block with an unspecified 20-bit prefix.

An IP address is part of a CIDR block, and is said to match the CIDR prefix if the initial N bits of the address and the CIDR prefix are the same. Thus, understanding CIDR requires that IP address be visualized in binary. The binary numeral system, or base-2 number system, is a Numeral system that represents numeric values using two symbols usually 0 and 1. Since the length of an IPv4 address is fixed at 32 binary bits, an N-bit CIDR prefix leaves 32 − N bits unmatched, and there are 2^{(32 − N)} possible combinations of these bits, meaning that 2^{(32 − N)} IPv4 addresses match a given N-bit CIDR prefix. Shorter CIDR prefixes match more addresses, while longer CIDR prefixes match fewer. An address can match multiple CIDR prefixes of different lengths.

CIDR is also used with IPv6 addresses, where the prefix length can range from 0 to 128, due to the larger number of bits in the address. Internet Protocol version 6 ( IPv6) is an Internet Layer protocol for packet -switched internetworks. A similar syntax is used: the prefix is written as an IPv6 address, followed by a slash and the number of significant bits

Assignment of CIDR blocks

The Internet Assigned Numbers Authority (IANA) issues to Regional Internet Registries (RIRs) large, short-prefix CIDR blocks. The Internet Assigned Numbers Authority (IANA is the entity that oversees global IP address allocation, DNS root zone management, media types A Regional Internet Registry ( RIR) is an organization overseeing the allocation and registration of Internet number resources within a particular region of the world For example, 62. 0. 0. 0/8, with over sixteen million addresses, is administered by RIPE NCC, the European RIR. The Réseaux IP Européens Network Coordination Centre ( RIPE NCC) is the Regional Internet Registry (RIR for Europe the Middle East and parts of Central Asia The RIRs, each responsible for a single, large, geographic area (such as Europe or North America), then subdivide these blocks into smaller blocks and issue them publicly. This subdividing process can be repeated several times at different levels of delegation. Large Internet service providers (ISPs) typically obtain CIDR blocks from an RIR, then subdivide them into smaller CIDR blocks for their subscribers, sized according to the size of the subscriber's network. An Internet service provider ( ISP, also called Internet access provider or IAP) is a company which primarily offers their customers access to the Internet Networks served by a single ISP are encouraged by IETF to obtain IP address space directly from their ISP. Networks served by multiple ISPs, on the other hand, will often obtain independent CIDR blocks directly from the appropriate RIR. Provider Independent Address Space (PI-addresses are Internet Protocol addresses assigned by Regional Internet Registries directly to an end-user organization without

For example, in the late 1990s, the IP address 208. 130. 29. 33 (since reassigned) was used by www. freesoft. org. An analysis of this address identified three CIDR prefixes. 208. 128. 0. 0/11, a large CIDR block containing over 2 million addresses, had been assigned by ARIN (the North American RIR) to MCI. The American Registry for Internet Numbers ( ARIN) is the Regional Internet Registry (RIR for Canada, the United States, and many island MCI Inc is an American Telecommunications company that is headquartered in Ashburn Virginia. Automation Research Systems, a Virginia VAR, leased an Internet connection from MCI and was assigned the 208. The Commonwealth of Virginia ( is an American state A value-added reseller (VAR is a company that adds some feature(s to an existing product(s then resells it (usually to end-users as an integrated product or complete " 130. 28. 0/22 block, capable of addressing just over 1000 devices. ARS used a /24 block for its publicly accessible servers, of which 208. 130. 29. 33 was one.

All of these CIDR prefixes would be used, at different locations in the network. Outside of MCI's network, the 208. 128. 0. 0/11 prefix would be used to direct to MCI traffic bound not only for 208. 130. 29. 33, but also for any of the roughly two million IP addresses with the same initial 11 bits. Within MCI's network, 208. 130. 28. 0/22 would become visible, directing traffic to the leased line serving ARS. Only within the ARS corporate network would the 208. 130. 29. 0/24 prefix have been used.

CIDR and masks

A subnet mask is a bitmask that encodes the prefix length in a form similar to an IP address: 32 bits, starting with a number of 1 bits equal to the prefix length, ending with 0 bits, and encoded in four-part dotted-decimal format. In Computer networks based on the Internet Protocol Suite, a subnetwork, or subnet, is a portion of the network's computers and network devices that have In Computer science, a mask is data that is used for Bitwise operations. A subnet mask encodes the same information as a prefix length, but predates the advent of CIDR.

CIDR uses variable length subnet masks (VLSM) to allocate IP addresses to subnets according to individual need, rather than some general network-wide rule. Thus the network/host division can occur at any bit boundary in the address. The process can be recursive, with a portion of the address space being further divided into even smaller portions, through the use of masks which cover more bits. Recursion, in Mathematics and Computer science, is a method of defining functions in which the function being defined is applied within its own definition

CIDR/VLSM network addresses are now used throughout the public Internet, although they are also used elsewhere, particularly in large private networks. An average desktop LAN user generally does not see them in practice, as their LAN is usually numbered using special private network addresses. In Internet terminology a private network is typically a network that uses private IP address space following the agreed standard of RFC 1918

Prefix aggregation

Another benefit of CIDR is the possibility of routing prefix aggregation (also known as "supernetting" or "route summarization"). Supernetting is synonymous with Classless Inter-Domain Routing (CIDR although CIDR is rather just the concept that is implemented when subnetting or For example, sixteen contiguous Class C (/24) networks could now be aggregated together, and advertised to the outside world as a single /20 route (if the first 20 bits of their network addresses match). Two aligned contiguous /20s could then be aggregated to a /19, and so forth. This allows a significant reduction in the number of routes that have to be advertised over the Internet, preventing 'routing table explosions' from overwhelming routers, and stopping the Internet from expanding further.

See IPv4 subnetting reference. Certain IPv4 address space regions are specially allocated and reserved for special uses such as Loopback interfaces Private networks (RFC 1918 and

(*) Note that for routed subnets bigger than /31 or /32, 2 needs to be subtracted from the number of available addresses - the largest address is used as the broadcast address, and typically the smallest address is used to identify the network itself. See RFC 1812 for more detail. It is also common for the gateway IP for that subnet to use an address, meaning that you would subtract 3 from the number of usable hosts that can be used on the subnet.

Historical background

IP addresses were originally separated into two parts: the network address (which identified a whole network or subnet), and the host address (which identified a particular machine's connection or interface to that network). An Internet Protocol ( IP) address is a numerical identification ( Logical address) that is assigned to devices participating in a Computer network This division was used to control how traffic was routed in and among IP networks. The Internet Protocol ( IP) is a protocol used for communicating data across a Packet-switched Internetwork using the Internet Protocol

Historically, the IP address space was divided into three main 'classes of network', where each class had a fixed size network address. Classful network is a term that is used to describe the network architecture of the Internet until around 1993. The class, and hence the length of the network address and the number of hosts on the network, could always be determined from the most significant bits of the IP address. Without any way of specifying a prefix length or a subnet mask, routing protocols, such as RIP-1, IGRP, necessarily used the class of the IP address specified in route advertisements to determine the size of the routing prefixes to be set up in the routing tables. Routing is the process of selecting paths in a network along which to send network traffic The Routing Information Protocol (RIP is a dynamic routing protocol used in local area networks Interior Gateway Routing Protocol ( IGRP) is a kind of IGP which is a Distance-vector routing protocol invented by Cisco, used by Routers In Computer networking a routing table, or Routing Information Base (RIB, is an electronic table (file or database type object that is stored in a

As the experimental TCP/IP network expanded into the Internet during the 1980s, the need for more flexible addressing schemes became increasingly apparent. This led to the successive development of subnetting and CIDR. In Computer networks based on the Internet Protocol Suite, a subnetwork, or subnet, is a portion of the network's computers and network devices that have Because the old class distinctions are ignored, the new system was called classless routing. It is supported by modern routing protocols, such as RIP-2, EIGRP, IS-IS and OSPF. A routing protocol is a protocol that specifies how Routers communicate with each other to disseminate information that allows them to select routes between any two The Routing Information Protocol (RIP is a dynamic routing protocol used in local area networks Enhanced Interior Gateway Routing Protocol - ( EIGRP) is a Cisco proprietary Routing protocol loosely based on their original IGRP. Intermediate system to intermediate system ( IS-IS) is a protocol used by network devices ( Routers to determine the best way to forward datagrams or packets through Open Shortest Path First ( OSPF) is a dynamic Routing protocol for use in Internet Protocol (IP networks This led to the original system being called, by back-formation, classful routing. In Etymology, back-formation refers to the process of creating a new Lexeme (less precisely a new "word" by removing actual or supposed Affixes Classful network is a term that is used to describe the network architecture of the Internet until around 1993.

Variable-Length Subnet Masking (VLSM) is the same concept as CIDR, but is mostly in historical usage.

Internet RFC 1338 was a major paradigm shift to establish a provider-based addressing and hierarchical routing. The Internet is a global system of interconnected Computer networks Hierarchical routing: Routing that is based on hierarchical addressing With the new RFC 1338-style provider-based supernetting, it was possible to create multiple hierarchical tiers and most tiers were envisioned to be internet service providers. Supernetting is synonymous with Classless Inter-Domain Routing (CIDR although CIDR is rather just the concept that is implemented when subnetting or An Internet service provider ( ISP, also called Internet access provider or IAP) is a company which primarily offers their customers access to the Internet Provider-based address space allocation was the new model, and BGP would evolve to BGP4, incorporating the RFC 1338 paradigm. The Border Gateway Protocol ( BGP) is the core routing protocol of the Internet. The Border Gateway Protocol ( BGP) is the core routing protocol of the Internet. For this shift to occur, the technique for supernetting-subnetting the IP address space required a modification. This new feature was called Classless Inter-Domain Routing (CIDR). (Note that RFC 1338 was replaced by RFC 1519) [1].

External links

• RFC 1518 - An Architecture for IP Address Allocation with CIDR
• RFC 4632 - Classless Inter-Domain Routing (CIDR): an Address Assignment and Aggregation Strategy
• RFC 2317 documents a way to do reverse DNS delegation for CIDR blocks
• RFC 4291 - IP Version 6 Addressing Architecture
• RFC 3021 - PtP links using /31 subnets (but /32 is more flexible)
• Online IP CIDR Calculator
• Freeware command-line subnet calculator for Windows and Linux
• Command-line calculator by Krischan Jodies (also in Debian)
• Grep-like command-line tool to match IP addresses by CIDR spec

Examples of New Address Assignment and Routing

• Refer to RFC 4632, Page 15, Section 6