CIDR notation
Encyclopedia
CIDR notation is a compact specification of an Internet Protocol address
and its associated routing prefix. Classless Inter-Domain Routing
(CIDR) is an Internet Protocol
(IP) address allocation and route aggregation methodology used within the Internet
addressing architecture that replaced the IPv4
classful network
organization of the IP address space. It is used also for IPv6
networking, the next generation of the IP addressing architecture.
CIDR notation is constructed from the IP address and the prefix size, the latter being the number of leading 1 bits of the routing prefix. The IP address is expressed according to the standards of IPv4 or IPv6. It is followed by a separator character, the slash (/) character, and the prefix size expressed as a decimal number.
The address may denote a single, distinct, interface address or the beginning address of an entire network. In the latter case the CIDR notation specifies the address block allocation of the network. The maximum size of the network is given by the number of addresses that are possible with the remaining, least-significant bits below the prefix. This is often called the host identifier.
For example:
Before CIDR notation, IPv4 networks usually used the dot-decimal notation
, an alternative representation which uses the network address followed by the network's subnet mask
. Thus, The CIDR notation 192.168.0.0/24 would be written as 192.168.0.0/255.255.255.0
The number of addresses of a subnet defined by the mask or prefix can be calculated as 2address size - mask, in which the address size for IPv6 is 128 and 32 for IPv4. For example, in IPv4, a mask of /29 gives: 232-29 = 23 = 8 addresses.
However, because at least one of these addresses is typically used for a gateway to other subnets, and because certain addresses are reserved as broadcast address
es, the number of addresses available for hosts is usually smaller.
IP address
An Internet Protocol address is a numerical label assigned to each device participating in a computer network that uses the Internet Protocol for communication. An IP address serves two principal functions: host or network interface identification and location addressing...
and its associated routing prefix. Classless Inter-Domain Routing
Classless Inter-Domain Routing
Classless Inter-Domain Routing is a method for allocating IP addresses and routing Internet Protocol packets. The Internet Engineering Task Force introduced CIDR in 1993 to replace the previous addressing architecture of classful network design in the Internet...
(CIDR) is an Internet Protocol
Internet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
(IP) address allocation and route aggregation methodology used within the Internet
Internet
The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...
addressing architecture that replaced the IPv4
IPv4
Internet Protocol version 4 is the fourth revision in the development of the Internet Protocol and the first version of the protocol to be widely deployed. Together with IPv6, it is at the core of standards-based internetworking methods of the Internet...
classful network
Classful network
A classful network is a network addressing architecture used in the Internet from 1981 until the introduction of Classless Inter-Domain Routing in 1993. The method divides the address space for Internet Protocol Version 4 into five address classes. Each class, coded in the first four bits of the...
organization of the IP address space. It is used also for IPv6
IPv6
Internet Protocol version 6 is a version of the Internet Protocol . It is designed to succeed the Internet Protocol version 4...
networking, the next generation of the IP addressing architecture.
CIDR notation is constructed from the IP address and the prefix size, the latter being the number of leading 1 bits of the routing prefix. The IP address is expressed according to the standards of IPv4 or IPv6. It is followed by a separator character, the slash (/) character, and the prefix size expressed as a decimal number.
The address may denote a single, distinct, interface address or the beginning address of an entire network. In the latter case the CIDR notation specifies the address block allocation of the network. The maximum size of the network is given by the number of addresses that are possible with the remaining, least-significant bits below the prefix. This is often called the host identifier.
For example:
- the address specification 192.168.100.1/24 represents the given IPv4IPv4Internet Protocol version 4 is the fourth revision in the development of the Internet Protocol and the first version of the protocol to be widely deployed. Together with IPv6, it is at the core of standards-based internetworking methods of the Internet...
address and its associated routing prefix 192.168.100.0, or equivalently, its subnet mask 255.255.255.0. - the IPv4IPv4Internet Protocol version 4 is the fourth revision in the development of the Internet Protocol and the first version of the protocol to be widely deployed. Together with IPv6, it is at the core of standards-based internetworking methods of the Internet...
block 192.168.0.0/22 represents the 1024 IPv4IPv4Internet Protocol version 4 is the fourth revision in the development of the Internet Protocol and the first version of the protocol to be widely deployed. Together with IPv6, it is at the core of standards-based internetworking methods of the Internet...
addresses from 192.168.0.0 to 192.168.3.255. - the IPv6IPv6Internet Protocol version 6 is a version of the Internet Protocol . It is designed to succeed the Internet Protocol version 4...
block 2001:DB8::/48 represents the IPv6 addresses from 2001:DB8:0:0:0:0:0:0 to 2001:DB8:0:FFFF:FFFF:FFFF:FFFF:FFFF. - ::1/128 represents the IPv6 loopbackLoopbackLoopback describes ways of routing electronic signals, digital data streams, or flows of items from their originating facility back to the source without intentional processing or modification...
address. Its prefix size is 128, i.e. the size of the address itself, indicating that this facility consists of only this one address.
Before CIDR notation, IPv4 networks usually used the dot-decimal notation
Dot-decimal notation
Dot-decimal notation is a presentation format for numerical data. It consists of a string of decimal numbers, each pair separated by a full stop ....
, an alternative representation which uses the network address followed by the network's subnet mask
Subnetwork
A subnetwork, or subnet, is a logically visible subdivision of an IP network. The practice of dividing a network into subnetworks is called subnetting....
. Thus, The CIDR notation 192.168.0.0/24 would be written as 192.168.0.0/255.255.255.0
The number of addresses of a subnet defined by the mask or prefix can be calculated as 2address size - mask, in which the address size for IPv6 is 128 and 32 for IPv4. For example, in IPv4, a mask of /29 gives: 232-29 = 23 = 8 addresses.
However, because at least one of these addresses is typically used for a gateway to other subnets, and because certain addresses are reserved as broadcast address
Broadcast address
A broadcast address is a logical address at which all devices connected to a multiple-access communications network are enabled to receive datagrams...
es, the number of addresses available for hosts is usually smaller.