Content-type: text/html Man page of CIDR


Section: Environments, Tables, and Troff Macros (7)
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CIDR, cidr - Classless Inter-Domain Routing  


Classless Inter-Domain Routing, also known as CIDR, is an Internet addressing architecture designed to solve two problems: the growth in the size of the routing tables in the top-level routers and the exhaustion of Class B address space. To solve these problems, CIDR relies on a new means of distributing the allocation of Internet address space and on a concept known as route aggregation.

For an up-to-date list of Frequently Asked Questions (FAQ) about CIDR, retrieve the CIDR FAQ from either of the following locations:  

Address Space Allocation

The traditional 32-bit Internet address is divided into a network part and a host part. The size of each part depends on the network class to which the address belongs. The following table shows the sizes of each part for Class A, Class B, and Class C networks.

NetworkSize of Network Part (in bits)Size of Host Part (in bits)

Class A824
Class B1616
Class C248

In this scenario, each physical network or LAN uses a single network number. While the idea seems sound, most organizations seldom create a single network containing thousands of hosts, choosing instead to divide their networks using routers. For organizations with Class B addresses, this wastes valuable addresses.

To cope with the various network topologies, the concept of subnetworks or subnetting emerged. In a subnet, the network part of the address consists of the network part and a portion of the host part. The bitmask convering these two parts is called the subnet mask. The area of the host part that is covered by the subnet mask identifies the subnet. This process allows you to identify individual LANS by their subnet number within the larger network number. The only way to communicate between two or more subnets is through a router.

Currently, routers make routing decisions by extracting the network portion of an IP address and looking it up in their routing table. This forces some IP routers to store each network number connected to the Internet in their routing table.

For many organizations, a Class C network (254 hosts) is too small, whereas a Class B network (65534 hosts) is too large, resulting in poor address space utilization.  

Route Aggregation

The Internet Advisory Board (IAB) and Internet Engineering Task Force (IETF) have decided to eliminate the notion of IP address classes and to direct routers to make routing decisions based on a variable-length, contiguous IP address prefix. This is what is meant by classless routing. Under this scenario, an Internet Service Provider (ISP) that had previously announced 256 contiguous Class C networks to the Internet, now only has to announce a single prefix, with 16 significant bits, for all these networks. This prefix is referred to as an aggregate, and the network is referred to as a supernet. If the ISP needed to add additional customers to its network, it could do so without modifying the routing announcements to the rest of the Internet.

Aggregating networks reduces the number of routers in a network and enables you to make optimum use of bridges and high-speed switches.  


This section describes one example of a Class C supernet. If organization A requires 1000 addresses, it might have the following Class C networks:,,, and Using current Class C addressing specifications, organization A's network mask and network numbers are as follows: /------------24 bits-----------\ 1111 1111 1111 1111 1111 1111 0000 0000 = mask 1101 0100 1101 1101 0010 0000 0000 0000 = network 1101 0100 1101 1101 0010 0001 0000 0000 = network 1101 0100 1101 1101 0010 0010 0000 0000 = network 1101 0100 1101 1101 0010 0011 0000 0000 = network \--------network address-------/ \--host--/
                                  address Typically, software compares all network address bits that are covered by the network mask (1 bits) to determine the effective network address. Because the network addresses covered by the 24-bit network mask are different, traffic from one network to another requires a router. In addition, routes to each of the four networks are advertised to the rest of the Internet, and occupy space in the routers' routing tables.

Under CIDR rules, organization A could shorten their network mask from 24 bits under current rules to 22 bits. The result is a network mask of, as follows: /-----------22 bits----------\ 1111 1111 1111 1111 1111 1100 0000 0000 = mask 1101 0100 1101 1101 0010 0000 0000 0000 = network 1101 0100 1101 1101 0010 0001 0000 0000 = network 1101 0100 1101 1101 0010 0010 0000 0000 = network 1101 0100 1101 1101 0010 0011 0000 0000 = network 1101 0100 1101 1101 0010 0011 0000 0000 = network \-------network address------/\----host----/
                                address Because the network addresses covered by the 22-bit network mask are the same, traffic from one network to another does not require a router. Instead, the software uses Address Resolution Protocol (ARP) to acquire direct connection to the network.

The address with the mask identifies all networks belonging to organization A. Expressed in CIDR format, organization A's network address is This effectively aggregates all routes under one network address. This also means that only one route is advertised to the rest of the Internet. If a router sees traffic addressed to with the netmask of, the traffic is addressed to network

Using a network mask of, organization A can have a single bridged network of 1022 hosts (hosts 0 and 1024 are reserved for the broadcast address). Using a network mask of, organization A can have two bridged networks of 510 hosts (host 0 and 512 are reserved for the broadcast address).

These techniques are not currently implemented in all host software, and should be implemented in networks with great care. However, the IETF suggests that host software be modified to allow for classless routing.  


Commands: netstat(1), ifconfig(8), route(8).

RFC1517, Applicability Statement for the Implementation of Classless Inter-Domain Routing (CIDR)

RFC1518, An Architecture for IP Address Allocation with CIDR

RFC1519, CIDR Address Strategy

RFC1520, Exchanging Routing Information Across Provider Boundaries in the CIDR Environment delim off



Address Space Allocation
Route Aggregation

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Time: 02:40:19 GMT, October 02, 2010