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Classless Inter-Domain Routing (CIDR /ˈsaɪdər, ˈsɪ-/) is a method for allocating IP addresses and IP routing. The Internet Engineering Task Force introduced CIDR in 1993 to replace the previous addressing architecture of classful network design in the Internet. Its goal was to slow the growth of routing tables on routers across the Internet, and to help slow the rapid exhaustion of IPv4 addresses.[1][2]

IP addresses are described as consisting of two groups of bits in the address: the most significant bits are the network prefix, which identifies a whole network or subnet, and the least significant set forms the host identifier, which specifies a particular interface of a host on that network. This division is used as the basis of traffic routing between IP networks and for address allocation policies.

Whereas classful network design for IPv4 sized the network prefix as one or more 8-bit groups, resulting in the blocks of Class A, B, or C addresses, Classless Inter-Domain Routing allocates address space to Internet service providers and end users on any address bit boundary. In IPv6, however, the interface identifier has a fixed size of 64 bits by convention, and smaller subnets are never allocated to end users.

CIDR encompasses several concepts. It is based on the variable-length subnet masking (VLSM) technique, which allows the specification of arbitrary-length prefixes. CIDR introduced a new method of representation for IP addresses, now commonly known as CIDR notation, in which an address or routing prefix is written with a suffix indicating the number of bits of the prefix, such as for IPv4, and 2001:db8::/32 for IPv6. CIDR introduced an administrative process of allocating address blocks to organizations based on their actual and short-term projected needs. The aggregation of multiple contiguous prefixes resulted in supernets in the larger Internet, which whenever possible are advertised as aggregates, thus reducing the number of entries in the global routing table.

Classless Inter-Domain Routing (CIDR) adalah metoda untuk mengalokasikan IP address dan routing paket Internet Protocol. CIDR di ajukan pada tahun 1993 untuk menggantikan cara pengalamatan yang lama yang menggunakan disain arsitektur classful network di Internet dengan tujuan untuk memperlambat pertumbuhan router yang ada di Internet, juga menunda kehabisan / kekurangan dari IPv4 address.

IP address biasanya terdiri dari dua group dari address: bagian most significant adalah network address yang mengidentifikasi seluruh jaringan atau subnet; sementara bagian least significant adalah host identifier, yang menerangkan host / interface yang tersambung ke jaringan. Cara pembagian ini yang menjadi dasar dari routing antara jaringan IP maupun kebijakan pengalokasian address. Rancangan classful network dari IPv4 membagi network address sebagai satu atau lebih 8-bit group, akibatnya kita kenal address kelas A, B dan C.

Classless Inter-Domain Routing mengalokasi address ke allocates address ke Internet service provider dan end user menggunakan batas address bit yang bebas, tidak menggunakan segmentasi 8 bit. Dalam IPv6, host identifier mempunya panjang yang tetap yaitu 64 bit, dan subnet yang lebih kecil tidak pernah di alokasikan ke end user.

Notasi CIDR menggunakan sintaks yang baru untuk menerangkan IP address IPv4 dan IPv6, yaitu dengan menggunakan base network address dilanjutkan dengan slash dan berarnya dari prefix routing, contoh, (IPv4), dan 2001:db8::/32 (IPv6).

Latar Belakang

Pada dekade pertama dari Internet setelah di temukannya Domain Name System (DNS) terlihat bahwa sistem yang berbasis kelas A, kelas B, dan keas C (biasa disebut classful network) dalam melakukan routing paket tidak scalable.

Untuk mengatasi keterbatasan tersebut, tahun 1993 Internet Engineering Task Force mempublikasikan beberapa standard, RFC 1518 dan RFC 1519, untuk mendefinisikan konsep baru dalam mengalokasikan blok IP address dan metoda baru untuk routing paket IPv4. Versi terbaru dari spesifikasi tersebut dipublikasikan sebagai RFC 4632 tahun 2006.

Sebuah IP address dapat dilihat sebagai dua bagian: sebuah prefix yang mengidentifikasi network yang di ikuti oleh identifikasi mesin / host dalam network tersebut. Dalam bahasa sederhana kita dapat melihat sebagai kode area (prefix) dan nomor pesawat telepon (host).

Pada arsitektur yang sebelumnya, alokasi IP address berdasarkan batasan 8 bit dalam 32 bit IP address, yang memaksa prefix:

  • prefix 8 bit, kelas A.
  • prefix 16 bit, kelas B.
  • prefix 24 bit, kelas C.
In the prior classful network architecture, IP address allocations were based on octet (8-bit) boundary segments of the 32-bit IP address, forcing either 8, 16, or 24-bit network prefixes. Thus, the smallest allocation and routing block contained only 256 addresses—too small for most enterprises, and the next larger block contained 65,536 addresses—too large to be used efficiently by even large organizations. This led to inefficiencies in address use as well as routing because the large number of allocated small (class-C) networks with individual route announcements, being geographically dispersed with little opportunity for route aggregation, created heavy demand on routing equipment.

Classless Inter-Domain Routing is based on variable-length subnet masking (VLSM) to allow allocation on arbitrary-length prefixes. Variable-length subnet masks are mentioned in RFC 950 (1985).

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. 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. This led to the original system being called, by back-formation, classful routing.

CIDR encompasses:

  • the VLSM technique of specifying arbitrary-length prefixes. An address in CIDR notation is written with a suffix indicating the number of bits in the prefix, such as, where /16 is the suffix, and is the prefix.
  • 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. Aggregation hides multiple levels of subnetting from the Internet routing table, and reverses the process of subnetting with VLSM.
  • the administrative process of allocating address blocks to organizations based on their actual and short-term projected need.

CIDR blocks

IP Address Match.jpg

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. These groups, commonly called CIDR blocks, share an initial sequence of bits in the binary representation of their IP addresses. 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 most significant bit of the address. When emphasizing only the size of a network, terms like /20 are used, which 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. Since the length of an IPv4 address has 32 bits, an N-bit CIDR prefix leaves 32-N bits unmatched, meaning that 232-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 and the syntax semantic is identical. A prefix length can range from 0 to 128, due to the larger number of bits in the address, however, by convention a subnet on broadcast MAC layer networks always has 64-bit host identifiers. Larger prefixes are rarely used even on point-to-point links.

Assignment of CIDR blocks

The Internet Assigned Numbers Authority (IANA) issues to Regional Internet Registries (RIRs) large, short-prefix (typically /8) CIDR blocks. For example,, with over sixteen million addresses, is administered by RIPE NCC, the European RIR. The RIRs, each responsible for a single, large, geographic area (such as Europe or North America), then subdivide these allocations into smaller blocks and issue them to local Internet registries. This subdividing process can be repeated several times at different levels of delegation. End user networks receive subnets sized according to the size of their network and projected short term need. Networks served by a single ISP are encouraged by IETF recommendations to obtain IP address space directly from their ISP. Networks served by multiple ISPs, on the other hand, may often obtain independent CIDR blocks directly from the appropriate RIR.

CIDR Address.jpg

For example, in the late 1990s, the IP address (since reassigned) was used by An analysis of this address identified three CIDR prefixes., a large CIDR block containing over 2 million addresses, had been assigned by ARIN (the North American RIR) to MCI. Automation Research Systems, a Virginia VAR, leased an Internet connection from MCI and was assigned the block, capable of addressing just over 1000 devices. ARS used a /24 block for its publicly accessible servers, of which was one.

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

Subnet masks

A subnet mask is a bitmask that encodes the prefix length in quad-dotted notation: 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. A subnet mask encodes the same information as a prefix length, but predates the advent of CIDR. However, in CIDR notation, the prefix bits are always contiguous, whereas subnet masks may specify non-contiguous bits. However, this has no practical advantage for increasing efficiency.

Agregasi Prefix

CIDR memungkinkan kita untuk memperoleh "agregasi prefix routing" yang lebih hasil, juga di kenal sebagai supernetting atau route summarization. Contoh, enam belas /24 network (dulu Kelas C) yang tersambung satu sama lain dapat di perkenalkan sebagai sebuah route /20 jika 20 bit pertama dari network address mereka sama persis. Dua network /20 yang berurutan dapat di gabung menjadi /19, dan seterusnya. Hal ini memungkinkan pengurangan yang sangat luar biasa dari route yang harus di perkenalkan ke jaringan.

IP/CIDR Δ ke IP address terakhir Mask Host (*) Kelas Catatan
a.b.c.d/32 + 1 1/256 C
a.b.c.d/31 + 2 1/128 C d = 0 ... (2n) ... 254
a.b.c.d/30 + 4 1/64 C d = 0 ... (4n) ... 252
a.b.c.d/29 + 8 1/32 C d = 0 ... (8n) ... 248
a.b.c.d/28 + 16 1/16 C d = 0 ... (16n) ... 240
a.b.c.d/27 + 32 1/8 C d = 0 ... (32n) ... 224
a.b.c.d/26 + 64 1/4 C d = 0, 64, 128, 192
a.b.c.d/25 + 128 1/2 C d = 0, 128
a.b.c.0/24 + 256 1 C
a.b.c.0/23 + 512 2 C c = 0 ... (2n) ... 254
a.b.c.0/22 + 1,024 4 C c = 0 ... (4n) ... 252
a.b.c.0/21 + 2,048 8 C c = 0 ... (8n) ... 248
a.b.c.0/20 + 4,096 16 C c = 0 ... (16n) ... 240
a.b.c.0/19 + 8,192 32 C c = 0 ... (32n) ... 224
a.b.c.0/18 + 16,384 64 C c = 0, 64, 128, 192
a.b.c.0/17 + 32,768 128 C c = 0, 128
a.b.0.0/16 + 65,536 256 C = 1 B
a.b.0.0/15 + 131,072 2 B b = 0 ... (2n) ... 254
a.b.0.0/14 + 262,144 4 B b = 0 ... (4n) ... 252
a.b.0.0/13 + 524,288 8 B b = 0 ... (8n) ... 248
a.b.0.0/12 + 1,048,576 16 B b = 0 ... (16n) ... 240
a.b.0.0/11 + 2,097,152 32 B b = 0 ... (32n) ... 224
a.b.0.0/10 + 4,194,304 64 B b = 0, 64, 128, 192
a.b.0.0/9 + 8,388,608 128 B b = 0, 128
a.0.0.0/8 + 16,777,216 256 B = 1 A
a.0.0.0/7 + 33,554,432 2 A a = 0 ... (2n) ... 254
a.0.0.0/6 + 67,108,864 4 A a = 0 ... (4n) ... 252
a.0.0.0/5 + 134,217,728 8 A a = 0 ... (8n) ... 248
a.0.0.0/4 + 268,435,456 16 A a = 0 ... (16n) ... 240
a.0.0.0/3 + 536,870,912 32 A a = 0 ... (32n) ... 224
a.0.0.0/2 + 1,073,741,824 64 A a = 0, 64, 128, 192
a.0.0.0/1 + 2,147,483,648 128 A a = 0, 128 + 4,294,967,296 256 A

(*) Catatan untuk route yang lebih besar dari /31 atau /32, 2 IP perlu di buang dari jumlah address yang tersedi - address terbesar biasanya digunakan sebagai broadcast address, sementara address terkecil biasanya digunakan untuk mengidentifikasi network itu sendiri / network address. Lihat RFC 1812 untu keterangan yang lebih rinci. Biasanya IP gateway akan mengambil satu address, akibatnya kita biasanya membuang tiga (3) address dari total IP dalam sebuah subnet yang dapat digunakan.


  • RFC 1518, An Architecture for IP Address Allocation with CIDR, Y. Rekhter, T. Li (Eds.), September 1993
  • RFC 1519, Classless Inter-Domain Routing (CIDR): an Address Assignment and Aggregation Strategy, V. Fuller, T. Li, J. Yu, K. Varadhan, September 1993
  • RFC 1517, Applicability Statement for the Implementation of Classless Inter-Domain Routing (CIDR), R. Hinden (Ed.), Internet Engineering Steering Group (September 1993)
  • RFC 4632, Classless Inter-domain Routing (CIDR): The Internet Address Assignment and Aggregation Plan, V. Fuller, T. Li, August 2006

Pranala Luar

  • RFC 2317 - Classless IN-ADDR.ARPA delegation
  • RFC 4291 - IP Version 6 Addressing Architecture
  • RFC 3021 - Using 31-Bit Prefixes on IPv4 Point-to-Point Links
  • CIDR Report (update setiap hari)

Pranala Menarik