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* CIDR
stands for Classless Inter-Domain Routing.
* CIDR
(Classless Inter-Domain Routing) was introduced in 1993 (RCF 1517) replacing the
previous generation of IP address syntax - classful networks. CIDR allowed for more
efficient use of IPv4 address space and prefix aggregation, known as route summarization
or supernetting.
* CIDR
introduction allowed for:
* More
efficient use of IPv4 address space
* Prefix
aggregation, which reduced the size of routing tables
* CIDR
allows routers to group routes together to reduce the bulk of routing information
carried by the core routers. With CIDR, several IP networks appear to
networks
outside the group as a single, larger entity. With CIDR, IP addresses and
their
subnet masks are written as four octets, separated by periods, followed by a
forward
slash and a two-digit number that represents the subnet mask
* CIDR
was developed in the 1990s as a standard scheme for routing network traffic across
the Internet.
* Before
CIDR technology was developed, Internet routers managed network traffic based
on the class of IP addresses. In this system, the value of an IP address
determines
its subnetwork for the purposes of routing.
* CIDR
is an alternative to traditional IP subnetting that organizes IP addresses into
subnetworks independent of the value of the addresses themselves. CIDR is also known
as supernetting as it effectively allows multiple subnets to be grouped
together
for network routing.
CIDR
Notation: - CIDR specifies an IP address range using a combination of an IP address
and its associated network mask. CIDR notation uses the following format –
xxx.xxx.xxx.xxx/n
where
n is the number of (leftmost) '1' bits in the mask. For example,
192.168.12.0/23
applies the network mask 255.255.254.0 to the 192.168 network,
starting
at 192.168.12.0. This notation represents the address range 192.168.12.0 –192.168.13.255.
Compared to traditional class-based networking, 192.168.12.0/23 represents an aggregation
of the two Class C subnets 192.168.12.0 and 192.168.13.0 each having a
subnet mask of 255.255.255.0. In other words,
192.168.12.0/23
= 192.168.12.0/24 + 192.168.13.0/24
Additionally,
CIDR supports Internet address allocation and message routing
independent
of the traditional class of a given IP address range. For example,
10.4.12.0/22
represents the address range 10.4.12.0 - 10.4.15.255 (network mask
255.255.252.0).
This allocates the equivalent of four Class C networks within the much larger
Class A space.
You
will sometimes see CIDR notation used even for non-CIDR networks. In non-CIDR
IP subnetting, however, the value of n is restricted to either 8 (Class
A), 16 (Class B) or 24
(Class
C). Examples:
* 10.0.0.0/8
* 172.16.0.0/16
* 192.168.3.0/24
CIDR
aggregation requires the network segments involved to be contiguous
(numerically adjacent) in the address space. CIDR cannot, for example,
aggregate 192.168.12.0 and 192.168.15.0 into a single route unless the
intermediate .13 and .14 address ranges are included (i.e., the
192.168.12/22 network).
With
CIDR, address classes (Class A, B, and C) became meaningless. The network
address
was no longer determined by the value of the first octet, but assigned prefix length
(subnet mask) address space. The number of hosts on a network, could now be assigned
a specific prefix depending upon the number of hosts needed for that network.
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