CIDR stands for Classless Inter-Domain Routing. CIDR was developed in the 1990s as a standard scheme for routing network traffic across the Internet.
Why Use CIDR?
Before CIDR technology was developed, Internet routers managed network traffic based on the class of IP addresses. In this system, the value of an IP address determines its subnetwork for the purposes of routing.
CIDR is an alternative to traditional IP subnettingthat 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 routing.
CIDR specifies an IP address range using a combination of an IP address and its associated network mask. CIDR notation uses the following format –
where n is the number of (leftmost) ‘1’ bits in the mask. For example,
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. In other words,
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,
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 subnetting, however, the value of n is restricted to either 8 (Class A), 16 (Class B) or 24 (Class C). Examples: