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Dated: Nov. 08, 2013
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Networking In GeneralThe CIDR notation is otherwise called Classless Inter-Domain Routing. It was first created back in the last decade of the previous century and it was a generic scheme that was used for routing data flows and traffic through the Internet.
Why should you use CIDR?
In the years before this technology was created, the way that routers handled traffic was through their class of their IP address. But, then came CIDR as a different approach to routing which did not use the classes of the IP addresses. CIDR it otherwise called supernetting because it enables many subents to form one group which helps routing.
CIDR Notation
The way CIDR works is that it combines the IP address and the associated network mask of that address. The format it uses is the following:
xxx.xxx.xxx.xxx/n
n stands for (leftmost) '1' bits in the mask. For example,
192.168.12.0/23
The Classless Inter-Domain Routing will also support address allocation and routing messages without the need to depend on normal IP classes.For example:
10.4.12.0/22
This stands for the range of the address 10.4.12.0 - 10.4.15.255 (if you consider that it has a network mask 255.255.252.0).
It is not very uncommon the see that somebody, somewhere is using a CIDR notation in a network than is not even CIDR.
How does CIDR work?
In order for CIDR to work properly it always needs support, it won't work without it. The protocols that supported it when it was created were OSPF and BGP. But, keep n mind that not every routing protocol will support CIDR, as older protocols may not be compatible with it.
It will also need parts of the network to be contiguous. If you don't understand let us give you an example. It can't aggregate two IP addresses into one route if there is no suitable address range included.
Backbone routers or wide area networks commonly support CIDR in order to save space where the addresses are. Normal household routers usually don't support it so it is not commonly found in home or private networks. Also, you will not see it often even in local area networks, only in wide area networks. This is all because of the fact that basic routers, which are the most common ones, do not support CIDR.
CIDR and IPv6
You will see that CIDR is often used by IPv6 exactly the same as IPv4. We should state that IPv6 is created in a way that it is completely without the need of classes, so it is completely classless.
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