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What Is CIDR Notation?

Classless inter-domain routing (CIDR) is a set of Internet protocol (IP) standards that is used to create unique identifiers for networks and individual devices. The IP addresses allow particular information packets to be sent to specific computers. Shortly after the introduction of CIDR, technicians found it difficult to track and label IP addresses, so a notation system was developed to make the process more efficient and standardized. That system is known as CIDR notation.

CIDR IP addresses consist of two groups of numbers, which are also referred to as groups of bits. The most important of these groups is the network address, and it is used to identify a network or a sub-network (subnet). The lesser of the bit groups is the host identifier. The host identifier is used to determine which host or device on the network should receive incoming information packets. In contrast to classful routing, which categorizes addresses into one of three blocks, CIDR allows for blocks of IP addresses to be allocated to Internet service providers. The blocks are then split up and assigned to the provider’s customers. Until recently, IP addresses used the IPv4 CIDR standard, but because IPv4 addresses are nearly exhausted, a new standard known as IPv6 has been developed and will soon be implemented.

Development of CIDR

When the Internet domain name system (DNS) was first established, the classful routing system was used for IP addresses, but early Internet developers soon discovered that it included a serious flaw in that it lacked scalability. To solve this problem, the Internet Engineering Task Force created the IPv4 standard in 1993. In addition, CIDR was created as a system of routing the new IPv4 addresses. These standards were originally published under the names RFC 1518 and RFC 1519. In 2006, a new version of the standard was published as RFC 4632.

According to the CIDR standard, the first part of an IP address is a prefix, which identifies the network. The prefix is followed by the host identifier so that information packets can be sent to particular computers within the network. With the classful routing system, individual networks were either limited to 256 host identifiers or overburdened with 65,536 identifiers. For many network enterprises, 256 identifiers were not enough and 65,536 were too burdensome to be used efficiently.

In the 1980s, as TCP/IP grew into the modern Internet, the need for a more flexible routing system was recognized. This need prompted the development of CIDR and subnets. CIDR and the process of variable-length subnet masking (VLSM) allow network administrators to divide individual networks into subnets of various sizes. In addition, addresses for related operations can be grouped together to create a simple system of categorization. Internet providers are also able to allocate a scalable number of addresses, in blocks, to organizations based on how many addresses are needed.

These new routing and categorization systems solved most of the problems with IP addresses, and the only remaining problem was deciding how to identify them efficiently. Eventually, CIDR notation was established and accepted as the standard. In CIDR notation, IP addresses are written as a prefix, and a suffix is attached to indicate how many bits are in the entire address. The suffix is set apart from the prefix with a slash mark. For instance, in the CIDR notation 192.0.1.0/24, the prefix is 192.0.1.0, and the total number of bits in the address is 24.

CIDR Blocks

The ability to group blocks of addresses into a single routing network is the hallmark of CIDR, and the prefix standard used for interpreting IP addresses makes this possible. CIDR blocks share the first part of the bit sequence that comprises the binary representation of the IP address, and blocks are identified using the same decimal-dot CIDR notation system that is used for IPv4 addresses. For example, 10.10.1.16/32 is an address prefix with 32 bits, which is the highest number of bits allowed in IPv4. Addresses with identical prefixes and the same number of bits always belong to the same block. In addition, larger blocks can be easily distinguished from smaller blocks by the length of the prefix. Short prefixes allow for more addresses while large prefixes identify small blocks.

CIDR notation is also used for the newer IPv6 standard, and the syntax is the same. The only difference is that IPv6 addresses may contain up to 128 bits instead of the 32-bit maximum of IPv4. Even though IPv6 addresses may be up to 128 bits in length, it is important to note that subnets on MAC layer networks always use 64-bit host identifiers.

The assignment of CIDR blocks is handled by the Internet Assigned Numbers Authority (IANA). One of the duties of the IANA is to issue large blocks of IP addresses to regional Internet registries (RIRs). These blocks are used for large geographical areas, such as Europe, North America, Africa and Australia. It is then the duty of each RIR to create smaller, but still quite large, blocks of IP addresses to be assigned to local Internet registries (LIRs). Depending on the organization of regional and local registries, blocks may be subdivided further until they are assigned to end users. The size of blocks assigned to end users is dependent on how many individual addresses will be required by each user. Most end users receive their blocks from a single Internet service provider (ISP), but organizations that make use of multiple ISPs must obtain provider-independent blocks directly from an LIR or RIR.

IPv4 CIDR IP/CIDRΔ to last IP addrMaskHosts (*)Class
a.b.c.d/32+0.0.0.0255.255.255.25511/256 C
a.b.c.d/31+0.0.0.1255.255.255.25421/128 C
a.b.c.d/30+0.0.0.3255.255.255.25241/64 C
a.b.c.d/29+0.0.0.7255.255.255.24881/32 C
a.b.c.d/28+0.0.0.15255.255.255.240161/16 C
a.b.c.d/27+0.0.0.31255.255.255.224321/8 C
a.b.c.d/26+0.0.0.63255.255.255.192641/4 C
a.b.c.d/25+0.0.0.127255.255.255.1281281/2 C
a.b.c.0/24+0.0.0.255255.255.255.0002561 C
a.b.c.0/23+0.0.1.255255.255.254.0005122 C
a.b.c.0/22+0.0.3.255255.255.252.0001,0244 C
a.b.c.0/21+0.0.7.255255.255.248.0012,0488 C
a.b.c.0/20+0.0.15.255255.255.240.0004,09616 C
a.b.c.0/19+0.0.31.255255.255.224.0008,19232 C
a.b.c.0/18+0.0.63.255255.255.192.00016,38464 C
a.b.c.0/17+0.0.127.255255.255.128.00032,768128 C
a.b.0.0/16+0.0.255.255255.255.000.00065,536256 C = 1 B
a.b.0.0/15+0.1.255.255255.254.000.000131,0722 B
a.b.0.0/14+0.3.255.255255.252.000.000262,1444 B
a.b.0.0/13+0.7.255.255255.248.000.000524,2888 B
a.b.0.0/12+0.15.255.255255.240.000.0001,048,57616 B
a.b.0.0/11+0.31.255.255255.224.000.0002,097,15232 B
a.b.0.0/10+0.63.255.255255.192.000.0004,194,30464 B
a.b.0.0/9+0.127.255.255255.128.000.0008,388,608128 B
a.0.0.0/8+0.255.255.255255.000.000.00016,777,216256 B = 1 A
a.0.0.0/7+1.255.255.255254.000.000.00033,554,4322 A
a.0.0.0/6+3.255.255.255252.000.000.00067,108,8644 A
a.0.0.0/5+7.255.255.255248.000.000.000134,217,7288 A
a.0.0.0/4+15.255.255.255240.000.000.000268,435,45616 A
a.0.0.0/3+31.255.255.255224.000.000.000536,870,91232 A
a.0.0.0/2+63.255.255.255192.000.000.0001,073,741,82464 A
a.0.0.0/1+127.255.255.255128.000.000.0002,147,483,648128 A
0.0.0.0/0+255.255.255.255000.000.000.0004,294,967,296256 A

* For routed subnets bigger than /31 or /32, two reserved addresses need to be subtracted from the number of available host addresses: the largest address, which is used as the broadcast address, and the smallest address, which is used to identify the network itself. In addition, any border router of a subnet typically uses a dedicated address.

Subnet Masks

Once blocks of IP addresses are assigned to end users, CIDR allows them to be further divided within a private network, which is a process known as subnetting. Computers and other connected devices within a particular subnet can be identified because they all use the same IP address prefix. The subnet identifier then becomes the most significant portion of the host identifier. Finally, the last part of the host identifier is used to distinguish individual computers on a subnet.

The subnet identifiers within a network are assigned according to the network’s subnet mask, which is a binary pattern that is used to determine how many subnets are available in a network. In its binary form, a subnet mask begins with a series of ones and ends with a series of zeros. However, subnet masks are usually expressed using the familiar dot-decimal notation used for IP addresses and network prefixes. In this notation, the series of ones become the number 255. For example, the most common subnet mask expressed using this notation is 255.255.255.0. This subnet mask is known as subnet zero, and it is used when only one subnet is required or as the first of multiple subnets.

A specific subnet mask is created by designating a portion of the host identifier, and larger subnets are created by moving more bits from the host identifier to the subnet mask. The final subnet of a network is designated in binary with all ones. When using the CIDR dot-decimal notation, the final subnet is expressed as 255.255.255.255.

Before CIDR, subnet masks with all zeros (255.255.255.0) and subnet masks with all ones (255.255.255.255) could not be used because they could become confused with network identifiers, but CIDR-compliant equipment uses the prefixes and suffixes of CIDR notation to distinguish between the two.

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