CIDR: Classless Inter-Domain Routing

Classless Inter-Domain Routing was developed in the 1990s as a standard scheme for routing network traffic across the internet. 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 IP subnetting. It organizes IP addresses into subnetworks independent of the value of the addresses themselves. CIDR is also known as supernetting because it effectively allows several subnets to be grouped together for network routing.

CIDR Notation

CIDR specifies an IP address range using a combination of an IP address and its associated network mask.

xxx.xxx.xxx.xxx/n

CIDR notation uses the above format, where n is the number of (leftmost) 1 bits in the mask.

192.168.12.0/23

The example above 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 to 192.168.13.255.

Compared to 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.

Here's another way to visualize it:

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.

10.4.12.0/22

The example above represents the address range 10.4.12.0 to 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'll 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).

Here are some examples:

• 10.0.0.0/8
• 172.16.0.0/16
• 192.168.3.0/24

How CIDR Works

When first implemented on the internet, the core routing protocols like Border Gateway Protocol and Open Shortest Path First were updated to support CIDR. Obsolete or less popular routing protocols might not support CIDR.

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.

All internet WAN or backbone routers — those that manage traffic between internet service providers — generally support CIDR to achieve the goal of conserving IP address space. Mainstream consumer routers often don't support CIDR, therefore private networks including home networks and even small public networks (LANs) often don't employ it.

CIDR and IPv6

IPv6 uses CIDR routing technology and CIDR notation in the same way as IPv4. IPv6 was designed for fully classless addressing.