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Classless Inter-Domain Routing (CIDR, or ) 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. 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.〔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)〕 IP addresses are described as consisting of two groups of bits in the address: the most significant bits are the ''network address'' (or ''network prefix'' or ''network block''), 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. 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 for specifying IP addresses and their associated routing prefix. It appends a slash character to the address and the decimal number of leading bits of the routing prefix, e.g., 192.168.2.0/24 for IPv4, and 2001:db8::/32 for IPv6. ==Background== During the first decade of the 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.〔RFC 1517, ''Applicability Statement for the Implementation of Classless Inter-Domain Routing (CIDR)'', R. Hinden, Ed., (September 1993)〕 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.〔RFC 4632, ''Classless Inter-domain Routing (CIDR): The Internet Address Assignment and Aggregation Plan'', V. Fuller, T. Li (August 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 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 initial TCP/IP network grew to become 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. The network class distinctions were removed, and the new system was described as being ''classless'', with respect to the old system, which became known as ''classful''. Classless Inter-Domain Routing is based on ''variable-length subnet masking'' (VLSM), which allows a network to be divided into variously sized subnets, providing the opportunity to size a network more appropriately for local needs. Variable-length subnet masks are mentioned in RFC 950.〔RFC 950, ''Internet Standard Subnetting Procedure'', J. Mogul, J. Postel, Eds. (August 1985), Section 2.1〕 Accordingly, techniques for grouping addresses for common operations were based on the concept of cluster addressing, first proposed by Carl-Herbert Rokitansky.〔Carl-Herbert Rokitansky, "Internet Cluster Addressing Scheme and its Application to Public Data Networks", Proc. 9th International Conference on Computer Communication (ICCC' 88), pp. 482-491, Tel Aviv, Israel, Oct./Nov. 1988〕〔(Cluster Addressing and CIDR ) in the mail archives of the IEFT〕 CIDR encompasses several concepts. It is based on the VLSM technique with effective qualities of specifying 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 192.168.2.0/24. 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. ==CIDR notation== CIDR notation is a compact representation of an IP address and its associated routing prefix. The notation is constructed from an IP address, a slash ('/') character, and a decimal number. The number is the count of leading ''1'' bits in the routing mask, traditionally called the network mask. The IP address is expressed according to the standards of IPv4 or IPv6. The address may denote a single, distinct interface address or the beginning address of an entire network. The maximum size of the network is given by the number of addresses that are possible with the remaining, least-significant bits below the prefix. The aggregation of these bits is often called the ''host identifier''. For example: *''192.168.100.14/24'' represents the IPv4 address 192.168.100.14 and its associated routing prefix 192.168.100.0, or equivalently, its subnet mask 255.255.255.0, which has 24 leading 1-bits. * the IPv4 block 192.168.100.0/22 represents the 1024 IPv4 addresses from 192.168.100.0 to 192.168.103.255. * the IPv6 block 2001:db8::/48 represents the block of IPv6 addresses from 2001:db8:0:0:0:0:0:0 to 2001:db8:0:ffff:ffff:ffff:ffff:ffff. * ''::1/128'' represents the IPv6 loopback address. Its prefix size is 128, the size of the address itself. Before the implementation of CIDR, IPv4 networks were represented by the starting address and the subnet mask, both written in dot-decimal notation. Thus, 192.168.100.0/24 was often written as 192.168.100.0/255.255.255.0. The number of addresses of a subnet may be calculated as 2address size - prefix size, in which the address size is 128 for IPv6 and 32 for IPv4. For example, in IPv4, the prefix size /29 gives: 232-29 = 23 = 8 addresses. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Classless Inter-Domain Routing」の詳細全文を読む スポンサード リンク
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