Non-Class Inter-Domain Routing ( CIDR ) is a method of allocating IP addresses and IP routing. The Internet Engineering Task Force introduced CIDR in 1993 to replace the previous addressing architecture of a classy network design on the Internet. The goal is to slow the growth of routing tables on routers on the Internet, and to help slow down the rapid fatigue of IPv4 addresses.
The IP address is described as consisting of two groups of bits in the address: the most significant bit is the network prefix, which identifies the entire network or subnet, and the least significant set forms the host identifier , which specifies the specific interface of the host in the network. This division is used as the basis of the traffic routing between IP networks and for address allocation policies.
While the classy network design for IPv4 measures the network prefix as one or more 8-bit groups, resulting in Class A, B or C address blocks, the Non-Class Interface Routing allocates the address space to the Internet service provider and the end user on each overcome the bit limit. However, in IPv6, interface identifiers have a fixed size of 64 bits by convention, and smaller subnets are never allocated to end users.
CIDR covers several concepts. This is based on the variable-length subnet masking ( VLSM ) technique, which allows arbitrary long-term prefix specification. CIDR introduces a new representation method for IP addresses, now commonly known as CIDR notation , where the address or routing prefix is ​​written with a suffix indicating the number of prefix bits, such as > 192.0.2.0 for IPv4, and 2001: db8 :: / 32 for IPv6. CIDR introduces the administrative process of allocating block addresses to organizations based on their actual and short-term projection needs. Aggregations of some adjacent prefixes produce supernets on the larger Internet, which, whenever possible, are advertised as aggregates, thereby reducing the number of entries in the global routing table.
Video Classless Inter-Domain Routing
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An IP address is defined as consisting of two parts: a prefix identifying a network followed by a host identifier within that network. In previous classful network architecture, IP address allocation is based on the bits of four octets of IP addresses. An address is considered to be a combination of a network prefix of 8, 16, or 24-bit along with a 24, 16, or 8-bit host identifier respectively. Thus, the smallest allocation and routing block contains only 256 addresses - too small for most companies, and the larger subsequent blocks containing addresses 65 536 - are too large for efficient use even by large organizations. This leads to inefficiencies in the use of addresses and inefficiencies in routing, as it takes a large number of C-class networks to be allocated with individual route announcements, which are geographically dispersed with little opportunity for route aggregation.
During the first decade of the Internet after the invention of the Domain Name System (DNS) it became clear that systems designed based on classful network schemes allocated IP address space and IP packet routing were not measurable. This leads to the development of subnetting and CIDR sequentially. Network class differences have been removed, and the new system is described as no class , in connection with the old system, which came to be known as classful . In 1993, the Internet Engineering Task Force published a new set of standards, RFC 1518 and RFCÃ, 1519, to define the concept of allocation of this new IP address block and the new method of IPv4 packet routing. The latest version of this specification was published as RFCÃ, 4632 in 2006.
Classless Inter-Domain Routing is based on variable-length subnet masking (VLSM), which allows the network to be split into multiple subnet sizes, providing an opportunity for more precise network sizes for local needs. The variable-length subnet mask is mentioned in RFC 950. Thus, the technique for classifying addresses for common operations is based on the concept of addressing clusters, first proposed by Carl-Herbert Rokitansky.
Maps Classless Inter-Domain Routing
notasi CIDR
CIDR notation is a compact representation of the IP address and the corresponding route prefix. This notation is built from IP addresses, slash characters ('/'), and decimal numbers. The number is the leading bit number 1 in the routing mask, which is traditionally called network mask. IP addresses are declared in accordance with IPv4 or IPv6 standards.
The address may indicate a different single interface address or a starting address of the entire network. The maximum network size is given by the number of possible addresses with the remaining bits, at least significantly below the prefix. Aggregation of these bits is often called host identifier .
As an example:
- 192.168.100.14 / 24 represents IPv4 addresses < i> 192.168.100.14 and the corresponding routing prefix 192.168.100.0 , or its equivalent subnet mask 255.255.255.0 , which has a prominent 24 bit 1.
- IPv4 block 192.168.100.0 represents 1024 IPv4 addresses from 192.168.100.0 to 192.168.103.255 .
- IPv6 block 2001: db8 :: / 48 represents the address block IPv6 from 2001: db8: 0: 0: 0: 0: 0: 0 to 2001: db8: 0: ffff: ffff: ffff: ffff: ffff .
- :: 1 / 128 represents the IPv6 loopback address. The length of the prefix is ​​128 which is the number of bits in the address.
Prior to CIDR implementation, the IPv4 network was represented by the initial address and subnet mask, both written in dot-decimal notation. Thus, 192.168.100.0 / 24 is often written as < span> 192.168.100.0 / 255.255.255.0 .
The number of subnet addresses can be counted as 2 address lengths - long prefix , where the address length is 128 for IPv6 and 32 for IPv4. For example, in IPv4, the long prefix / 29 gives: 2 32 - 29 = 2 3 = 8 addresses.
Subnet mask
The subnet mask is a bitmask that encodes a prefix length in quadrilateral notation: 32 bits, starting with 1 bit equal to prefix length, ending with 0 bits, and encoded in four decimal sections:.255.0 . The subnet mask encodes the same information as the prefix length, but precedes the CIDR. In CIDR notation, the prefix bit is always adjacent. The subnet mask is permitted by RFC 950 to determine non-contiguous bits until RFC 4632 states that the mask should be left side by side. Given this constraint, the subnet mask and CIDR notation serve the exact same function.
Block CIDR
CIDR is basically a bitwise, prefix-based standard for representation of IP addresses and their routing properties. This facilitates routing by allowing address blocks to be grouped into single routing table entries. These groups, usually called CIDR blocks, share the initial sequence of bits in the binary representation of their IP addresses. The IPv4 CIDR block is identified using a syntax similar to the IPv4 address: decimal address, followed by a slash, then a number from 0 to 32, ie abcd / n . The dotted decimal part is an IPv4 address. The number that follows the slash is the length of the prefix, the number of initial bits together, which is calculated from the most significant address bits. When emphasizing only the network size, the address part of the notation is usually omitted. Thus, the /span> 20 block is a CIDR block with an unspecified 20-bit prefix.
The IP address is part of the CIDR block, and is said to match the CIDR prefix if the initial bit n of the address and CIDR prefix are the same. The IPv4 address is 32 bits so the prefix n -bit CIDR leaves 32 - n unmatched bits, which means 2 32 - n The IPv4 address matches the given n -bit CIDR prefix. Shorter CIDR prefix matches more addresses, while older prefixes match fewer. An address can match multiple CIDR prefixes with different lengths.
CIDR is also used for IPv6 addresses and syntax identical semantics. The length of the prefix can range from 0 to 128, due to the larger number of bits in the address. However, by convention, subnets on a broadcast MAC layer network always have 64-bit host identifiers. Larger prefixes are rarely used even on point-to-point links.
Assignment of CIDR blocks
Internet Assigned Numbers Authority (IANA) issues into large regional Internet blocks (RIRs), short CIDR block prefixes. For example, 62.0.0.0 / 8 , with over sixteen million addresses , managed by RIPE NCC, European RIR. RIRs, each responsible for one, large, geographic area, such as Europe or North America, divide these blocks and allocate subnets to the local Internet registry (LIR). Similar divisions can be repeated several times at lower delegation levels. End user networks receive subnets that are of a size appropriate to their network size and project short term requirements. Networks served by one ISP are encouraged by IETF recommendations to get IP address space directly from their ISP. Networks served by multiple ISPs, on the other hand, can obtain a provider-independent address space directly from the corresponding RIR.
For example, in the late 1990s, the IP address 208.130.29.33 (since moved) was used by www.freesoft.org. This address analysis identifies three CIDR prefixes. 208.128.0.0 / 11 , a large CIDR block containing more than 2 million address, has been assigned by ARIN (North American RIR) to MCI. The Automation Research System, Virginia VAR, leases Internet connections from MCI and is given 208.130.28.0 / 22 block, capable of handling more than 1000 devices. ARS uses the /span> 24 block for a publicly accessible server, where < span> 208.130.29.33 is one. All of these CIDR prefixes will be used, in different locations within the network. Outside of the MCI network, the 208.128.0.0 / 11 will be used to redirect to MCI traffic bound not only to 208.130.29.33 , but also to about two million IP addresses with the same initial 11 bits. In the MCI network, 208.130.28.0 / 22 will be visible, directing traffic to the leased line serving ARS. Only in ARS corporate networks, the prefix 208.130.29.0 / 24 has been used.
Block IPv4 CIDR
In common usage, the first address in the subnet, all binary zeros in the host identifier, is reserved to refer to the network itself, while the last address, all binaries in the host identifier, is used as the broadcast address for the network; this reduces the number of addresses available to hosts by 2. As a result, the network / 31 , with one binary digit in host identifier, is rarely used, since such subnets will not provide the host address available after this reduction. RFC 3021 creates an exception for the "host all ones" and "host all zero" rules to create a / 31 network which can be used for point-to-point links. But in practice, point-to-point links are still usually implemented using / 30 networks, with / 31 is preferred by some providers. / 32 The address must be accessed by explicit routing rules, since there is no space on the network for the gateway single).
In a flattened subnet greater than 31 or / 32 , the number of available host addresses is typically minus two, ie the largest address, reserved as the broadcast address, and the smallest address, which identifies the network itself.
Block IPv6 CIDR
The large address size used in IPv6 enables implementing worldwide route routing and ensures a sufficient set of addresses on each site. The standard subnet size for IPv6 networks is the /span> 64 block, which is required for address operations without automatic configuration state. Initially, the IETF was recommended in RFC 3177 as a best practice that all end sites received the address / 48 However, , criticism and re-evaluation of actual needs and practices have led to more flexible allocation recommendations in RFC 6177 suggesting allocations that are significantly smaller for some sites, such as / 56 blocking for home network. This IPv6 subwoofer reference specifies sizes for IPv6 subnets. Different types of network links may require different subnet sizes. The subnet mask separates the bits from the network identifier prefix of the interface identifier bits. Selecting smaller prefix sizes results in fewer network numbers, but with more addresses in the network.
2001: 0db8: 0123: 4567: 89ab: cdef: 1234: 5678 |||| |||| |||| |||| |||| |||| |||| |||| |||| |||| |||| |||| |||| |||| |||| ||| 128 Single endpoint and loopback |||| |||| |||| |||| |||| |||| |||| ||| 127 Point-to-point links (inter-router) |||| |||| |||| |||| |||| |||| |||| || 124 |||| |||| |||| |||| |||| |||| |||| | 120 |||| |||| |||| |||| |||| |||| |||| 116 |||| |||| |||| |||| |||| |||| ||| 112 |||| |||| |||| |||| |||| |||| || 108 |||| |||| |||| |||| |||| |||| | 104 |||| |||| |||| |||| |||| |||| 100 |||| |||| |||| |||| |||| ||| 96 |||| |||| |||| |||| |||| || 92 |||| |||| |||| |||| |||| | 88 |||| |||| |||| |||| |||| 84 |||| |||| |||| |||| ||| 80 |||| |||| |||| |||| || 76 |||| |||| |||| |||| | 72 |||| |||| |||| |||| 68 |||| |||| |||| ||| 64 Single LAN; default prefix size for SLAAC |||| |||| |||| || 60 Multiple (very limited) 6th deployments (/60 = 16/64) |||| |||| |||| | 56 The minimal site end task; eg home network (/56 = 256/64) |||| |||| |||| 52/52 blocks = 4096/64 blocks |||| |||| ||| 48 Typical assignments for larger sites (/48 = 65536/64) |||| |||| || 44 |||| |||| | 40 |||| |||| 36 possible registration of local registry (LIR) extra-small in the future |||| ||| 32 minimum allocations of LIR |||| || 28 LIR medium allocations |||| | 24 LIR large allocation |||| 20 LIR extra large allocations ||| 16 || 12 Registry of Regional Internet Registry (RIR) from IANA 8 4
Prefix aggregation
CIDR provides a smooth route prefix aggregation. For example, sixteen connecting/24 networks can be combined and advertised to a larger network as single/20 routing table entries, if the first 20 bits of their network prefix match. Two adjacent blocks/20 blocks can be combined as/19 networks. This reduces the number of routes to be advertised.
See also
- Internet protocol suite
References
External links
- RFCÃ, 2317, Direction IN -ADDR.ARPA without class
- CIDR report (updated daily)
- CIDR FAQ
Source of the article : Wikipedia
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