CIDR stands for Classless Inter-Domain Routing (occasionally, Classless Internet Domain Routing). CIDR was developed in the 1990s as a standard scheme for routing network traffic across the Internet.
CIDR is an alternative to traditional IP sub netting that organizes IP addresses into subnetworks independent of the value of the addresses themselves. CIDR is also known as supernetting as it effectively allows multiple subnets to be grouped together for network.
CIDR reduced the problem of wasted address space by providing a new and more flexible way to specify network addresses in routers. CIDR lets one routing table entry represent an aggregation of networks that exist in the forward path that don't need to be specified on that particular gateway. This is much like how the public telephone system uses area codes to channel calls toward a certain part of the network. This aggregation of networks in a single address is sometimes referred to as a super net.
Using CIDR, each IP address has a network prefix that identifies either one or several network gateways. The length of the network prefix in IPv4 CIDR is also specified as part of the IP address and varies depending on the number of bits needed, rather than any arbitrary class assignment structure.
A destination IP address or route that describes many possible destinations has a shorter prefix and is said to be less specific. A longer prefix describes a destination gateway more specifically. Routers are required to use the most specific, or longest, network prefix in the routing table when forwarding packets. (In IPv6, a CIDR block always gets 64 bits for specifying network addresses.)
CIDR Notation
CIDR specifies an IP address range using a combination of an IP address and its associated network mask. CIDR notation uses the following format -
i. xxx.xxx.xxx.xxx/n
where n is the number of (leftmost) '1' bits in the mask. For example,
ii. 192.168.12.0/23
applies the network mask 255.255.254.0 to the 192.168 network, starting at 192.168.12.0. This notation represents the address range 192.168.12.0 - 192.168.13.255. Compared to traditional class-based networking, 192.168.12.0/23 represents an aggregation of the two Class C subnets 192.168.12.0 and 192.168.13.0 each having a subnet mask of 255.255.255.0.
iii. 192.168.12.0/23 = 192.168.12.0/24 + 192.168.13.0/24
Additionally, CIDR supports Internet address allocation and message routing independent of the traditional class of a given IP address range. For example,
iv. 10.4.12.0/22
represents the address range 10.4.12.0 - 10.4.15.255 (network mask 255.255.252.0). This allocates the equivalent of four Class C networks within the much larger Class A space.
You will sometimes see CIDR notation used even for non-CIDR networks. In non-CIDR IP sub netting, however, the value of n is restricted to either 8 (Class A), 16 (Class B) or 24 (Class C). Examples:
i. 10.0.0.0/8
ii. 172.16.0.0/16.
iii. 192.168.3.0/24
CIDR implementations
i. CIDR implementations require certain support be embedded within the network routing protocols. When first implemented on the Internet, the core routing protocols like BGP (Border Gateway Protocol) and OSPF (Open Shortest Path First) were updated to support CIDR. Obsolete or less popular routing protocols may not support CIDR.
ii. CIDR aggregation requires the network segments involved to be contiguous (numerically adjacent) in the address space. CIDR cannot, for example, aggregate 192.168.12.0 and 192.168.15.0 into a single route unless the intermediate .13 and .14 address ranges are included (i.e., the 192.168.12/22 network).
Internet WAN or backbone routers (those that manage traffic between Internet Service Providers) all generally support CIDR to achieve the goal of conserving IP address space. Mainstream consumer routers often do not support CIDR, therefore private networks (including home networks) and even small public networks (LANs) often do not employ it
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