Classless Inter-Domain Routing (CIDR)
Classless Inter-Domain Routing is a method for allocating IP addresses and for IP routing. The Internet Engineering Task Force introduced CIDR in 1993 to replace the previous classful network addressing architecture on 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.
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, under CIDR address space is allocated 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 variable-length subnet masking (VLSM) 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 192.0.2.0/24 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.
Classful Network Routing (deprecated)
A classful network is a network addressing architecture used in the Internet from 1981 until the introduction of Classless Inter-Domain Routing in 1993.
The method divides the IP address space for Internet Protocol version 4 (IPv4) into five address classes based on the leading four address bits. Classes A, B, and C provide unicast addresses for networks of three different network sizes. Class D is for multicast networking and the class E address range is reserved for future or experimental purposes.
Since its discontinuation, remnants of classful network concepts have remained in practice only in limited scope in the default configuration parameters of some network software and hardware components, most notably in the default configuration of subnet masks.
Class A Network [/ 8 Prefixes]
The first class, designated as Class A, contained all addresses in which the most significant bit is zero [0...]. The network number for this class is given by the next 7 bits, therefore accommodating 128 networks in total, including the zero network, and including the IP networks already allocated.
|IP Range:||0.0.0.0 to 127.255.255.255|
|Binary:||00000000.00000000.00000000.00000000 to 01111111.11111111.11111111.11111111|
RED = Class 'A' significant bit; RED/GREEN = Network bits; BLUE = Address bits
|Mask:||255.0.0.0 (8 bits)|
|Addresses:||16,777,216 (Per Network)|
|Hosts:||16,777,214 (Per Network)|
Class B Networks [/16 Prefixes]
A Class B network was a network in which all addresses had the two most-significant bits set to 1 and 0 respectively [10...]. For these networks, the network address was given by the next 14 bits of the address, thus leaving 16 bits for numbering host on the network for a total of 65,536 addresses per network.
|IP Range:||22.214.171.124 to 126.96.36.199|
|Binary:||10000000.00000000.00000000.00000000 to 10111111.11111111.11111111.11111111|
RED = Class 'B' significant bits; RED/GREEN = Network bits; BLUE = Address bits
|Mask:||255.255.0.0 (16 bits)|
|Addresses:||65,536 (Per Network)|
|Hosts:||65,534 (Per Network)|
Class C Networks [/24 Prefixes]
Class C was defined with the 3 high-order bits set to 1, 1, and 0, [110...] and designating the next 21 bits to number the networks, leaving each network with 256 local addresses.
|IP Range:||192.0.0.0 to 188.8.131.52|
|Binary:||11000000.00000000.00000000.00000000 to 11011111.11111111.11111111.11111111|
RED = Class 'C' significant bits; RED/GREEN = Network bits; BLUE = Address bits
|Mask:||255.255.255.0 (24 bits)|
|Addresses:||256 (Per Network)|
|Hosts:||254 (Per Network)|
Class D and E Networks
The leading bit sequence 111 designated an at-the-time unspecified addressing mode ("escape to extended addressing mode"), which was later subdivided as Class D (1110) for multicast addressing, while leaving as reserved for future use the 1111 block designated as Class E.