Classless Inter-Domain Routing

Classless Inter- Domain Routing (CIDR) describes a method for the efficient use of the existing 32- bit IP address space for IPv4. It was introduced in 1993 (RFC 1518, RFC 1519, RFC 4632 ) to reduce the size of routing tables and to view the available address ranges exploit better.

With CIDR, the fixed assignment of an IPv4 address no longer has to a network class, from which emerged the prefix length. By additionally specifying a network mask, the IP address is divided into network and host portions now.

In CIDR notation is introduced as a new so-called suffixes. The suffix indicates the number of 1 bits in the netmask. This form of writing, such as 172.17.0.0/17, is much shorter and easier than dealing Dotted decimal notation as 172.17.0.0/255.255.128.0 and also unique.

With IPv6, the notation is the same as in CIDR in IPv4 or IPv6 address consists of and prefix length (for example, 2001:0 DB8: 0: CD30 :: 1/60 ).

Examples

  • The notation 192.168.2.7/24 corresponds to the address 192.168.2.7 with the subnet mask 255.255.255.0: In binary notation of the netmask is 11111111.11111111.11111111.00000000 - so there are 3 x 8 = 24 bits set, as specified in the suffix.

Calculation

IPv4 address = 10.43.8.67/28 (29 to 32 bits = 4 (Netmask: 11111111.11111111.11111111.11110000 ( 4 digits in the binary system 16 different values ​​can be represented, namely 0-15) ) → 16 addresses - (broadcast and network address ) = 14 to assign IPv4 addresses. )

Netmask = 255.255.255.240

Others

CIDR also offers route aggregation. Different networks can be addressed under a single address.

The routing protocols BGP, RIP v2 (RIP II) (not RIP / RIP I) and OSPF and the Cisco proprietary EIGRP have implemented CIDR.

The CIDR notation has since become a broad-based for referencing IP networks in application software (eg in IRC daemon ). Thus, it is possible an access list to apply to all IPv4 addresses from 172.25.0.0 to 172.25.255.255 to write briefly as 172.25.0.0/16. Occasionally there will be less significant octets that are zero are omitted, so that recorded for 172.25.0.0/16 also 172.25/16 - although this is not specified in RFC 4632 anchored and therefore not recommended.

Survey

The following table shows the number of addresses per subnet and usable for hosts addresses, minus the addresses of a range for the network address that is ( all bits in the host part set to zero ) and the broadcast address ( all bits in the host portion to one).

Since the introduction of CIDR is classful routing although virtually abolished, to denote, for example, a / 24 network as a "Class C", however, is at least colloquially, preserved - even though this term usually is even wrong because now former Class A or Class B networks are allocated as smaller allocations / assignments and thus you possibly speak of a "Class C" major network, which would be a subnet of a Class A or B network according to the classical notation.

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