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CIDR: Classless Inter-Domain Routing

The basics on CIDR notation and IP addresses

By Bradley Mitchell Bradley Mitchell Writer Massachusetts Institute of Technology University of Illinois An MIT graduate who brings years of technical experience to articles on SEO, computers, and wireless networking. lifewire's editorial guidelines Updated on April 12, 2020 Tweet Share Email Tweet Share Email Home Networking The Wireless Connection Routers & Firewalls Network Hubs ISP Broadband Ethernet Installing & Upgrading Wi-Fi & Wireless 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: 10.0.0.0/8172.16.0.0/16192.168.3.0/24

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 implementations require certain support to be embedded within the network routing protocols. 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. Was this page helpful? Thanks for letting us know! Get the Latest Tech News Delivered Every Day Subscribe Tell us why! Other Not enough details Hard to understand Submit More from Lifewire 255.255.255.0 Subnet Mask for IP Networks Why There Are Only 13 DNS Root Name Servers IP Tutorial: Subnet Mask and Subnetting Private IP Addresses: Everything You Need to Know What Is an IP Address? How to Find the Default IP Address of a Belkin Router What Is a Public IP Address? (and How to Find Yours) How to Connect Two Routers on a Home Network Can't Connect to the Internet? Try These 10 Tips 127.0.0.1 IP Address Explained DNS Servers: What Are They and Why Are They Used? How to Fix an IPv6 No Network Access Error How to Change DNS Servers in Windows How to Look up an IP Address Owner How to Change Your IP Address What Happened to IPv5? Newsletter Sign Up Newsletter Sign Up Newsletter Sign Up Newsletter Sign Up Newsletter Sign Up By clicking “Accept All Cookies”, you agree to the storing of cookies on your device to enhance site navigation, analyze site usage, and assist in our marketing efforts. Cookies Settings Accept All Cookies