- Classless Inter-Domain Routing
Classless Inter-Domain Routing (CIDR) is a method for allocating IP addresses and routing Internet Protocol packets. The Internet Engineering Task Force introduced CIDR in 1993 to replace the previous addressing architecture of classful network design in the Internet. Their goal was to slow the growth of routing tables on routers across the Internet, and to help slow the rapid exhaustion of IPv4 addresses.
IP addresses are described as consisting of two groups of bits in the address: the most significant part is the network address which identifies a whole network or subnet and the least significant portion is 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. Classful network design for IPv4 sized the network address 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, instead of on 8-bit segments. 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 notation is a syntax of specifying IP addresses and their associated routing prefix. It appends to the address a slash character and the decimal number of leading bits of the routing prefix, e.g., 192.168.0.0/16 for IPv4, and 2001:db8::/32 for IPv6.
During the first decade of the modern Internet after the invention of the Domain Name System (DNS) it became apparent that the devised system based on the classful network scheme of allocating the IP address space and the routing of IP packets was not scalable.
To alleviate the shortcomings, the Internet Engineering Task Force published in 1993 a new set of standards, RFC 1518 and RFC 1519, to define a new concept of allocation of IP address blocks and new methods of routing IPv4 packets. A new version of the specification was published as RFC 4632 in 2006.
An IP address is interpreted as composed of two parts: a network-identifying prefix followed by a host identifier within that network. In the previous classful network architecture of Internet Protocol Version 4, IP address allocations were based on the bit boundaries of the four octets of an IP address. An address was considered to be the combination of an 8, 16, or 24-bit network prefix along with a 24, 16, or 8-bit individual or node address. 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.
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.
Classless Inter-Domain Routing is based on variable-length subnet masking (VLSM), which allows a network to be divided into different-sized subnets. CIDR avoids wasting IP addresses. Variable-length subnet masks are mentioned in RFC 950. 
CIDR encompasses several concepts. It is based on the VLSM technique with effective qualities of specifying arbitrary-length prefixes. Addresses are represented in CIDR notation, in which an address or routing prefix is written with a suffix indicating the number of bits in the address, such as 192.168.0.0/16. 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 when ever possible are advertised as aggregates, thus reducing the number of entries in the global routing table.
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 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, the address portion of the notation is usually 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. 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 prefixes match fewer. An address can match multiple CIDR prefixes of different lengths.
CIDR is also used for 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 CIDR blocks. For example, 126.96.36.199/8, 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 blocks 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 obtain provider-independent address space directly from the appropriate RIR.
For example, in the late 1990s, the IP address 188.8.131.52 (since reassigned) was used by www.freesoft.org. An analysis of this address identified three CIDR prefixes. 184.108.40.206/11, 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 220.127.116.11/22 block, capable of addressing just over 1000 devices. ARS used a /24 block for its publicly accessible servers, of which 18.104.22.168 was one.
All of these CIDR prefixes would be used, at different locations in the network. Outside of MCI's network, the 22.214.171.124/11 prefix would be used to direct to MCI traffic bound not only for 126.96.36.199, but also for any of the roughly two million IP addresses with the same initial 11 bits. Within MCI's network, 188.8.131.52/22 would become visible, directing traffic to the leased line serving ARS. Only within the ARS corporate network would the 184.108.40.206/24 prefix have been used.
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.
CIDR provides the possibility of fine-grained routing prefix aggregation, also known as supernetting or route summarization. For example, sixteen contiguous /24 networks can be aggregated and advertised to a larger network as a single /20 route, if the first 20 bits of their network addresses match. Two aligned contiguous /20s may then be aggregated to a /19, and so forth. This allows a significant reduction in the number of routes that have to be advertised.
IPv4 CIDR IP/CIDR Δ to last IP addr Mask Hosts (*) Class Notes a.b.c.d/32 +0.0.0.0 255.255.255.255 1 1/256 C a.b.c.d/31 +0.0.0.1 255.255.255.254 2 1/128 C d = 0 ... (2n) ... 254 a.b.c.d/30 +0.0.0.3 255.255.255.252 4 1/64 C d = 0 ... (4n) ... 252 a.b.c.d/29 +0.0.0.7 255.255.255.248 8 1/32 C d = 0 ... (8n) ... 248 a.b.c.d/28 +0.0.0.15 255.255.255.240 16 1/16 C d = 0 ... (16n) ... 240 a.b.c.d/27 +0.0.0.31 255.255.255.224 32 1/8 C d = 0 ... (32n) ... 224 a.b.c.d/26 +0.0.0.63 255.255.255.192 64 1/4 C d = 0, 64, 128, 192 a.b.c.d/25 +0.0.0.127 255.255.255.128 128 1/2 C d = 0, 128 a.b.c.0/24 +0.0.0.255 255.255.255.000 256 1 C a.b.c.0/23 +0.0.1.255 255.255.254.000 512 2 C c = 0 ... (2n) ... 254 a.b.c.0/22 +0.0.3.255 255.255.252.000 1,024 4 C c = 0 ... (4n) ... 252 a.b.c.0/21 +0.0.7.255 255.255.248.000 2,048 8 C c = 0 ... (8n) ... 248 a.b.c.0/20 +0.0.15.255 255.255.240.000 4,096 16 C c = 0 ... (16n) ... 240 a.b.c.0/19 +0.0.31.255 255.255.224.000 8,192 32 C c = 0 ... (32n) ... 224 a.b.c.0/18 +0.0.63.255 255.255.192.000 16,384 64 C c = 0, 64, 128, 192 a.b.c.0/17 +0.0.127.255 255.255.128.000 32,768 128 C c = 0, 128 a.b.0.0/16 +0.0.255.255 255.255.000.000 65,536 256 C = 1 B a.b.0.0/15 +0.1.255.255 255.254.000.000 131,072 2 B b = 0 ... (2n) ... 254 a.b.0.0/14 +0.3.255.255 255.252.000.000 262,144 4 B b = 0 ... (4n) ... 252 a.b.0.0/13 +0.7.255.255 255.248.000.000 524,288 8 B b = 0 ... (8n) ... 248 a.b.0.0/12 +0.15.255.255 255.240.000.000 1,048,576 16 B b = 0 ... (16n) ... 240 a.b.0.0/11 +0.31.255.255 255.224.000.000 2,097,152 32 B b = 0 ... (32n) ... 224 a.b.0.0/10 +0.63.255.255 255.192.000.000 4,194,304 64 B b = 0, 64, 128, 192 a.b.0.0/9 +0.127.255.255 255.128.000.000 8,388,608 128 B b = 0, 128 a.0.0.0/8 +0.255.255.255 255.000.000.000 16,777,216 256 B = 1 A a.0.0.0/7 +220.127.116.11 254.000.000.000 33,554,432 2 A a = 0 ... (2n) ... 254 a.0.0.0/6 +18.104.22.168 252.000.000.000 67,108,864 4 A a = 0 ... (4n) ... 252 a.0.0.0/5 +22.214.171.124 248.000.000.000 134,217,728 8 A a = 0 ... (8n) ... 248 a.0.0.0/4 +126.96.36.199 240.000.000.000 268,435,456 16 A a = 0 ... (16n) ... 240 a.0.0.0/3 +188.8.131.52 224.000.000.000 536,870,912 32 A a = 0 ... (32n) ... 224 a.0.0.0/2 +184.108.40.206 192.000.000.000 1,073,741,824 64 A a = 0, 64, 128, 192 a.0.0.0/1 +127.255.255.255 128.000.000.000 2,147,483,648 128 A a = 0, 128 0.0.0.0/0 +255.255.255.255 000.000.000.000 4,294,967,296 256 A
(*) For routed subnets bigger than /31 or /32, two reserved addresses need to be subtracted from the number of available host addresses: the largest address, which is used as the broadcast address, and the smallest address, which is used to identify the network itself.  It is also common for the IP gateway for that subnet to use an address, meaning that three addresses would have to be subtracted from the number of hosts that can be used on the subnet.
- ^ RFC 1518, An Architecture for IP Address Allocation with CIDR, Y. Rekhter, T. Li (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., (September 1993)
- ^ RFC 4632, Classless Inter-domain Routing (CIDR): The Internet Address Assignment and Aggregation Plan, V. Fuller, T. Li (August 2006)
- ^ RFC 950, Internet Standard Subnetting Procedure, J. Mogul, J. Postel, Eds. (August 1985), Section 2.1
- ^ RFC 922, Broadcasting Internet Datagrams in the Presence of Subnets, J. Mogul (Ed.), October 1984, Section 7.
- ^ RFC 1812, Requirements for IP Version 4 Routers, F. Baker (Ed.), June 1995, Section 220.127.116.11
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